user1040: A long time ago I had a romantic relationship with a married woman, and later I regretted. How is this seen in Buddhism?

user1053: The key thing is that you've stopped and regretted the unwholesome action. Practice diligently, do lots of wholesome actions and move on. It's those who keep making the same mistake that will place themselves one step closer to the bad destination:

"Bhikkhus, the bhikkhuni, ... re ... the novice in training, ... re ... the novice for higher ordination, ... re ... the female novice for higher ordination, ... re ... the lay disciple male, ... re ... the lay disciple female endowed with five things, is in hell as though led and lain there. What five?
Destroying living things, taking the not given, misbehaving in sexual desires, telling lies and taking intoxicating and brewed drinks. Bhikkhus, the bhikkhu endowed with these five things, is in hell as though led and lain there." ~~ ( AN 5.291/292: http://www.metta.lk/tipitaka/2Sutta-Pitaka/4Anguttara-Nikaya/Anguttara3/5-pancakanipata/026-upasampadavaggo-e.html )


user1046: The bottom line is whether anybody was harmed -- the husband, the wife, children, anyone else. (To depend solely on the literal sexual conduct code from ancient times is to risk mixing in cultural factors that have changed, often significantly; the harm criterion is universal.)
If there was harm, then the Mahayana doctrine of the Four Powers (often called the Four Opponent Powers) can be used to purify the negative karma. (I don't know the citations, but I'm quite sure this doctrine is derived from early sutras.) The Four Powers are:

The power of regret for having done the negative action.
The power of reliance: taking refuge, which restores our relationship with holy objects, and generating the altruistic intention, which restores our relationship with other sentient beings.
The power of the remedial action, e.g. prostration, offering, reciting the names of the Buddha, reading or contemplating the Dharma, etc.
The power of the promise not to repeat the action.

This is from http://archive.thubtenchodron.org/PrayersAndPractices/purification_practices.html, but there are many sites and references. 
Most discussions of the Four Powers are in a Mahayana or even Vajrayana context, but I see no reason they cannot be used by Theravada, Zen or Pure Land practitioners. Refuge is common to all Buddhist traditions; metta meditation could be used in place of the Mahayana "altruistic intention"; and a wide variety of practices or actions can be used for remedial action if done in the context of the other powers.
With regard to #1, regret, note that this is not heavy guilt or a sense of permanent, unpurifiable "sin", but a simple recognition that the action was wrong and needs remediation. In fact, the very possibility of purification opposes a sense of heavy guilt. Regret, in this sense, is the first power because it is clearly a prerequisite to any meaningful purification.
It sounds like you already have healthy regret and are resolved not to repeat such actions, and so are already well started on the full purification practice!

user16515: You have done something that that completely rails against the eightfold path, which was designed to end greed, ignorance, aversion, delusion. An affair waters all of these roots, however, you realized you have done something wrong. 
Fix yourself, continue the path.

user533: I would like to touch an important point: It is not because there was no explicit harm (being caught or maybe a divorce) that no harm was made at all, this is not how karma works, at least in most traditions, for sure in Theravada and Tibetan Buddhism.
Of course you are not condemned in a sense that you will necessarily go to hell, however the bad karma was created with or without explicit harm or being caught, the seed is now there, you created the cause for future suffering, you broke the 4th precept (assuming you knew she was married).
Ok, what is done is done, what can you do know? The Buddha gave a great simile that can help us here, the simile of water and salt.
The Blessed one said: Imagine if you drop salt in a cup of water, how will the water taste? Very bad of course. Now imagine if you drop the same amount of salt in a river, how will the river water taste? Probably good! The salt represents bad karma and water the good karma, so try to make as much good karma as you can, take the precepts, live a moral life etc.
The story of Angulimala proves us that nothing is impossible to revert. Looks like coming here and making this question can be a promissing start!

user43: "and later I regretted" -- remorse is a klesha, an obscuring emotion. Have you learned your lesson and decided what you will do differently now -- that's what matters for your growth, the constructive part, not the feeling of guilt which only weighs you down.

user-1: 
Am I condemned for being romantically involved with another man's wife?

Condemned by...

the community you live in? If this is unaccepted behavior, yes.

by the law? If that was unlawful, yes.

by Buddha? He is not here, so I highly doubt it.



How is this seen in Buddhism?

Short answer: it is seen as a "don't do it". If you did, it is seen as "well, don't do it again".
long answer: the Saleyaka Sutta (MN 41) reads:

[...] When [the brahman householders of Sala] were seated, they said to the Blessed One: "Master Gotama, what is the reason, what is the condition, why some beings here, on the dissolution of the body, after death, reappear in states of deprivation, in an unhappy destination, in perdition, even in hell; and what is the reason, what is the condition, why some beings here, on the dissolution of the body, after death, reappear in a happy destination, even in the heavenly world?"
"Householders, it is by reason of conduct not in accordance with the Dhamma, by reason of unrighteous conduct, that beings here on the dissolution of the body, after death, reappear in states of deprivation, in an unhappy destination, in perdition, even in hell. It is by reason of conduct in accordance with the Dhamma, by reason of righteous conduct, that some beings here on the dissolution of the body, after death, reappear in a happy destination, even in the heavenly world."

and then he says:

"And how are there three kinds of bodily conduct not in accordance with the Dhamma, unrighteous conduct? Here someone [...] is given over to misconduct in sexual desires: he has intercourse with such (women) as are protected by the mother, father, (mother and father), brother, sister, relatives, as have a husband, as entail a penalty, and also with those that are garlanded in token of betrothal. That is how there are three kinds of bodily conduct not in accordance with the Dhamma, unrighteous conduct.

The above, however, does not denote a condemnation as an external invisible force that will seek to punish you. But there are consequences to actions, according to the nature of the actions themselves. In this sense, one could consider these as the condemnation.
In the Maha Kammavibhanga Sutta, the Buddha (MN 136) says:

"Ananda, there are four kinds of persons existing in the world. What four?
(i) "Here some person kills living beings, takes what is not given, misconducts himself in sexual desires, speaks falsehood, speaks maliciously, speaks harshly, gossips, is covetous, is ill-willed, and has wrong view. On the dissolution of the body, after death, he reappears in the states of deprivation, in an unhappy destination, in perdition, in hell.
(ii) "But here some person kills living beings... and has wrong view. On the dissolution of the body, after death, he reappears in a happy destination, in the heavenly world.
(iii) "Here some person abstains from killing living beings, from taking what is not given, from misconduct in sexual desires, from false speech, from malicious speech, from harsh speech, from gossip, he is not covetous, is not ill-willed, and has right view. On the dissolution of the body, after death, he reappears in a happy destination, in the heavenly world.
(iv) "But here some person abstains from killing living beings... and has right view. On the dissolution of the body, after death, he reappears in the states of deprivation, in an unhappy destination, in perdition, in hell.

His explanation for a person who has performed misconducts to reappear in a good destination is given:

[...] "Now there is the person who has killed living beings here ... has had wrong view. And on the dissolution of the body, after death, he reappears in a happy destination, in the heavenly world. But (perhaps) the good kamma producing his happiness was done by him earlier, or the good kamma producing his happiness was done by him later, or right view was undertaken and completed by him at the time of his death. And that was why, on the dissolution of the body, after death, he reappeared in a happy destination, in the heavenly world. But since he has killed living beings here... has had wrong view, he will feel the result of that here and now, or in his next rebirth, or in some subsequent existence.

Beings are, as he says, heirs of their karma, and under certain conditions, karma  manifests its effects, sooner or later. That is, until one strives for and finally reaches nirvana.

user745: It is classified as sexual misconduct. According to the Cunda Kammaraputa Sutta:

And how is one made impure in three ways by bodily action? There is
  the case where a certain person takes life, is a hunter,
  bloody-handed, devoted to killing & slaying, showing no mercy to
  living beings. He takes what is not given. He takes, in the manner of
  a thief, things in a village or a wilderness that belong to others and
  have not been given by them. He engages in sensual misconduct. He
  gets sexually involved with those who are protected by their mothers,
  their fathers, their brothers, their sisters, their relatives, or
  their Dhamma; those with husbands, those who entail punishments,
  or even those crowned with flowers by another man. This is how one is
  made impure in three ways by bodily action.
http://www.accesstoinsight.org/tipitaka/an/an10/an10.176.than.html

However, that doesn't mean that you are "condemned." You did something that wasn't right, but if you recognize that you did something wrong and strive to do better in the future, that's all you should do. In the Samaññaphala Sutta King Ajatasattu confessed to the Buddha that he had murdered his own father, which in Buddhism is one of 5 Garu Kammas, the 5 most heinous acts a person could possibly commit:

"...King Ajatasattu said to the Blessed One: "Magnificent, lord! Magnificent! Just as if he were to place upright what was overturned,
  to reveal what was hidden, to show the way to one who was lost, or to
  carry a lamp into the dark so that those with eyes could see forms, in
  the same way has the Blessed One — through many lines of reasoning —
  made the Dhamma clear. I go to the Blessed One for refuge, to the
  Dhamma, and to the community of monks. May the Blessed One remember me
  as a lay follower who has gone to him for refuge, from this day
  forward, for life.
"A transgression has overcome me, lord, in that I was so foolish, so muddle-headed, and so unskilled as to kill my father — a righteous
  man, a righteous king — for the sake of sovereign rulership. May the
  Blessed One please accept this confession of my transgression as such,
  so that I may restrain myself in the future."
"Yes, great king, a transgression overcame you in that you were so foolish, so muddle-headed, and so unskilled as to kill your father — a
  righteous man, a righteous king — for the sake of sovereign rulership.
  But because you see your transgression as such and make amends in
  accordance with the Dhamma, we accept your confession. For it is a
  cause of growth in the Dhamma & Discipline of the noble ones when,
  seeing a transgression as such, one makes amends in accordance with
  the Dhamma and exercises restraint in the future."
       http://www.accesstoinsight.org/tipitaka/dn/dn.02.0.than.html

The Buddha did not condemn King Ajatasattu for his evil act, but encouraged him to have a change of heart and live an ammended life. He did the same for Angulimala the Serial Killer (Angulimala, his nickname, means garland of fingers because he kept his victim's fingers on a string as a garland) having him become a Monk and when Angulimala saw a woman in childbirth and wanted to help, the Buddha said this to ease Angulimala's feelings of guilt over killing so many people:

"In that case, Angulimala, go to that woman and on arrival say to her,
  'Sister, since I was born I do not recall intentionally killing a
  living being. Through this truth may there be wellbeing for you,
  wellbeing for your fetus.'"
"But, lord, wouldn't that be a lie for me? For I have intentionally
  killed many living beings."
"Then in that case, Angulimala, go to that woman and on arrival say to
  her, 'Sister, since I was born in the noble birth, I do not recall
  intentionally killing a living being. Through this truth may there be
  wellbeing for you, wellbeing for your fetus.'"
http://www.accesstoinsight.org/tipitaka/mn/mn.086.than.html

The Buddha taught people guilty of the most heinous things to change their ways but to move on and not dwell on their previous errors. If the Buddha encouraged a man guilty of killing his father and a serial killer not to dwell on guilt, how much more would he encourage someone who committed adultery to simply have a change of heart and not dwell on guilt. 

1040: <p>A long time ago I had a romantic relationship with a married woman, and later I regretted. How is this seen in Buddhism?</p>
 1053: <p>The key thing is that you've stopped and regretted the unwholesome action. Practice diligently, do lots of wholesome actions and move on. It's those who keep making the same mistake that will place themselves one step closer to the bad destination:</p>
<blockquote>
<p>&quot;Bhikkhus, the bhikkhuni, ... re ... the novice in training, ... re ... the novice for higher ordination, ... re ... the female novice for higher ordination, ... re ... the lay disciple male, ... re ... the lay disciple female endowed with five things, is in hell as though led and lain there. What five?</p>
<p>Destroying living things, taking the not given, misbehaving in sexual desires, telling lies and taking intoxicating and brewed drinks. Bhikkhus, the bhikkhu endowed with these five things, is in hell as though led and lain there.&quot; ~~ ( AN 5.291/292: <a href="http://www.metta.lk/tipitaka/2Sutta-Pitaka/4Anguttara-Nikaya/Anguttara3/5-pancakanipata/026-upasampadavaggo-e.html" rel="nofollow noreferrer">http://www.metta.lk/tipitaka/2Sutta-Pitaka/4Anguttara-Nikaya/Anguttara3/5-pancakanipata/026-upasampadavaggo-e.html</a> )</p>
</blockquote>
 1046: <p>The bottom line is whether anybody was harmed -- the husband, the wife, children, anyone else. (To depend solely on the literal sexual conduct code from ancient times is to risk mixing in cultural factors that have changed, often significantly; the harm criterion is universal.)</p>

<p>If there was harm, then the Mahayana doctrine of the Four Powers (often called the Four Opponent Powers) can be used to purify the negative karma. (I don't know the citations, but I'm quite sure this doctrine is derived from early sutras.) The Four Powers are:</p>

<ol>
<li><p>The power of regret for having done the negative action.</p></li>
<li><p>The power of reliance: taking refuge, which restores our relationship with holy objects, and generating the altruistic intention, which restores our relationship with other sentient beings.</p></li>
<li><p>The power of the remedial action, e.g. prostration, offering, reciting the names of the Buddha, reading or contemplating the Dharma, etc.</p></li>
<li><p>The power of the promise not to repeat the action.</p></li>
</ol>

<p>This is from <a href="http://archive.thubtenchodron.org/PrayersAndPractices/purification_practices.html" rel="nofollow">http://archive.thubtenchodron.org/PrayersAndPractices/purification_practices.html</a>, but there are many sites and references. </p>

<p>Most discussions of the Four Powers are in a Mahayana or even Vajrayana context, but I see no reason they cannot be used by Theravada, Zen or Pure Land practitioners. Refuge is common to all Buddhist traditions; metta meditation could be used in place of the Mahayana "altruistic intention"; and a wide variety of practices or actions can be used for remedial action if done in the context of the other powers.</p>

<p>With regard to #1, regret, note that this is not heavy guilt or a sense of permanent, unpurifiable "sin", but a simple recognition that the action was wrong and needs remediation. In fact, the very possibility of purification opposes a sense of heavy guilt. Regret, in this sense, is the first power because it is clearly a prerequisite to any meaningful purification.</p>

<p>It sounds like you already have healthy regret and are resolved not to repeat such actions, and so are already well started on the full purification practice!</p>
 16515: <p>You have done something that that completely rails against the eightfold path, which was designed to end greed, ignorance, aversion, delusion. An affair waters all of these roots, however, you realized you have done something wrong. </p>

<p>Fix yourself, continue the path.</p>
 533: <p>I would like to touch an important point: It is not because there was no explicit harm (being caught or maybe a divorce) that no harm was made at all, this is not how karma works, at least in most traditions, for sure in Theravada and Tibetan Buddhism.</p>

<p>Of course you are not condemned in a sense that you will necessarily go to hell, however the bad karma was created with or without explicit harm or being caught, the seed is now there, you created the cause for future suffering, you broke the 4th precept (assuming you knew she was married).</p>

<p>Ok, what is done is done, what can you do know? The Buddha gave a great simile that can help us here, the simile of water and salt.</p>

<p>The Blessed one said: Imagine if you drop salt in a cup of water, how will the water taste? Very bad of course. Now imagine if you drop the same amount of salt in a river, how will the river water taste? Probably good! The salt represents bad karma and water the good karma, so try to make as much good karma as you can, take the precepts, live a moral life etc.</p>

<p>The story of Angulimala proves us that nothing is impossible to revert. Looks like coming here and making this question can be a promissing start!</p>
 43: <p>"and later I regretted" -- remorse is a <em>klesha</em>, an obscuring emotion. Have you learned your lesson and decided what you will do differently now -- that's what matters for your growth, the constructive part, not the feeling of guilt which only weighs you down.</p>
 -1: <blockquote>
<p>Am I condemned for being romantically involved with another man's wife?</p>
</blockquote>
<p>Condemned by...</p>
<ol>
<li><p>the community you live in? If this is unaccepted behavior, yes.</p>
</li>
<li><p>by the law? If that was unlawful, yes.</p>
</li>
<li><p>by Buddha? He is not here, so I highly doubt it.</p>
</li>
</ol>
<blockquote>
<p>How is this seen in Buddhism?</p>
</blockquote>
<p><strong>Short answer</strong>: it is seen as a &quot;don't do it&quot;. If you did, it is seen as &quot;well, don't do it again&quot;.</p>
<p><strong>long answer</strong>: the Saleyaka Sutta (<a href="http://www.accesstoinsight.org/tipitaka/mn/mn.041.nymo.html" rel="nofollow noreferrer">MN 41</a>) reads:</p>
<blockquote>
<p>[...] When [the brahman householders of Sala] were seated, they said to the Blessed One: &quot;Master Gotama, what is the reason, what is the condition, why some beings here, on the dissolution of the body, after death, reappear in states of deprivation, in an unhappy destination, in perdition, even in hell; and what is the reason, what is the condition, why some beings here, on the dissolution of the body, after death, reappear in a happy destination, even in the heavenly world?&quot;</p>
<p>&quot;Householders, it is by reason of conduct not in accordance with the Dhamma, by reason of unrighteous conduct, that beings here on the dissolution of the body, after death, reappear in states of deprivation, in an unhappy destination, in perdition, even in hell. It is by reason of conduct in accordance with the Dhamma, by reason of righteous conduct, that some beings here on the dissolution of the body, after death, reappear in a happy destination, even in the heavenly world.&quot;</p>
</blockquote>
<p>and then he says:</p>
<blockquote>
<p>&quot;And how are there three kinds of bodily conduct not in accordance with the Dhamma, unrighteous conduct? Here someone [...] is given over to misconduct in sexual desires: he has intercourse with such (women) as are protected by the mother, father, (mother and father), brother, sister, relatives, <strong>as have a husband</strong>, as entail a penalty, and also with those that are garlanded in token of betrothal. That is how there are three kinds of bodily conduct not in accordance with the Dhamma, unrighteous conduct.</p>
</blockquote>
<p>The above, however, does not denote a condemnation as an external invisible force that will seek to punish you. But there are consequences to actions, according to the nature of the actions themselves. In this sense, one could consider these as the condemnation.</p>
<p>In the Maha Kammavibhanga Sutta, the Buddha (<a href="http://www.accesstoinsight.org/tipitaka/mn/mn.136.nymo.html" rel="nofollow noreferrer">MN 136</a>) says:</p>
<blockquote>
<p>&quot;Ananda, there are four kinds of persons existing in the world. What four?</p>
<p>(i) &quot;Here some person kills living beings, takes what is not given, misconducts himself in sexual desires, speaks falsehood, speaks maliciously, speaks harshly, gossips, is covetous, is ill-willed, and has wrong view. On the dissolution of the body, after death, he reappears in the states of deprivation, in an unhappy destination, in perdition, in hell.</p>
<p>(ii) &quot;But here some person kills living beings... and has wrong view. On the dissolution of the body, after death, he reappears in a happy destination, in the heavenly world.</p>
<p>(iii) &quot;Here some person abstains from killing living beings, from taking what is not given, from misconduct in sexual desires, from false speech, from malicious speech, from harsh speech, from gossip, he is not covetous, is not ill-willed, and has right view. On the dissolution of the body, after death, he reappears in a happy destination, in the heavenly world.</p>
<p>(iv) &quot;But here some person abstains from killing living beings... and has right view. On the dissolution of the body, after death, he reappears in the states of deprivation, in an unhappy destination, in perdition, in hell.</p>
</blockquote>
<p>His explanation for a person who has performed misconducts to reappear in a good destination is given:</p>
<blockquote>
<p>[...] &quot;Now there is the person who has killed living beings here ... has had wrong view. And on the dissolution of the body, after death, he reappears in a happy destination, in the heavenly world. But (perhaps) the good kamma producing his happiness was done by him earlier, or the good kamma producing his happiness was done by him later, or right view was undertaken and completed by him at the time of his death. And that was why, on the dissolution of the body, after death, he reappeared in a happy destination, in the heavenly world. But since he has killed living beings here... has had wrong view, he will feel the result of that here and now, or in his next rebirth, or in some subsequent existence.</p>
</blockquote>
<p>Beings are, as he says, heirs of their karma, and under certain conditions, karma  manifests its effects, sooner or later. That is, until one strives for and finally reaches nirvana.</p>
 745: <p>It is classified as sexual misconduct. According to the Cunda Kammaraputa Sutta:</p>

<blockquote>
  <p>And how is one made impure in three ways by bodily action? There is
  the case where a certain person takes life, is a hunter,
  bloody-handed, devoted to killing &amp; slaying, showing no mercy to
  living beings. He takes what is not given. He takes, in the manner of
  a thief, things in a village or a wilderness that belong to others and
  have not been given by them. He engages in sensual misconduct. <em>He
  gets sexually involved with those who are protected by their mothers,
  their fathers, their brothers, their sisters, their relatives, or
  their Dhamma; <strong>those with husbands</strong>, those who entail punishments,
  or even those crowned with flowers by another man.</em> This is how one is
  made impure in three ways by bodily action.</p>
  
  <p><a href="http://www.accesstoinsight.org/tipitaka/an/an10/an10.176.than.html">http://www.accesstoinsight.org/tipitaka/an/an10/an10.176.than.html</a></p>
</blockquote>

<p>However, that doesn't mean that you are "condemned." You did something that wasn't right, but if you recognize that you did something wrong and strive to do better in the future, that's all you should do. In the Samaññaphala Sutta King Ajatasattu confessed to the Buddha that he had murdered his own father, which in Buddhism is one of 5 Garu Kammas, the 5 most heinous acts a person could possibly commit:</p>

<blockquote>
  <p>"...King Ajatasattu said to the Blessed One: "Magnificent, lord! Magnificent! Just as if he were to place upright what was overturned,
  to reveal what was hidden, to show the way to one who was lost, or to
  carry a lamp into the dark so that those with eyes could see forms, in
  the same way has the Blessed One — through many lines of reasoning —
  made the Dhamma clear. I go to the Blessed One for refuge, to the
  Dhamma, and to the community of monks. May the Blessed One remember me
  as a lay follower who has gone to him for refuge, from this day
  forward, for life.</p>
  
  <p>"A transgression has overcome me, lord, in that I was so foolish, so muddle-headed, and so unskilled as to kill my father — a righteous
  man, a righteous king — for the sake of sovereign rulership. May the
  Blessed One please accept this confession of my transgression as such,
  so that I may restrain myself in the future."</p>
  
  <p>"Yes, great king, a transgression overcame you in that you were so foolish, so muddle-headed, and so unskilled as to kill your father — a
  righteous man, a righteous king — for the sake of sovereign rulership.
  But because you see your transgression as such and make amends in
  accordance with the Dhamma, we accept your confession. For it is a
  cause of growth in the Dhamma &amp; Discipline of the noble ones when,
  seeing a transgression as such, one makes amends in accordance with
  the Dhamma and exercises restraint in the future."
       <a href="http://www.accesstoinsight.org/tipitaka/dn/dn.02.0.than.html">http://www.accesstoinsight.org/tipitaka/dn/dn.02.0.than.html</a></p>
</blockquote>

<p>The Buddha did not condemn King Ajatasattu for his evil act, but encouraged him to have a change of heart and live an ammended life. He did the same for Angulimala the Serial Killer (Angulimala, his nickname, means garland of fingers because he kept his victim's fingers on a string as a garland) having him become a Monk and when Angulimala saw a woman in childbirth and wanted to help, the Buddha said this to ease Angulimala's feelings of guilt over killing so many people:</p>

<blockquote>
  <p>"In that case, Angulimala, go to that woman and on arrival say to her,
  'Sister, since I was born I do not recall intentionally killing a
  living being. Through this truth may there be wellbeing for you,
  wellbeing for your fetus.'"</p>
  
  <p>"But, lord, wouldn't that be a lie for me? For I have intentionally
  killed many living beings."</p>
  
  <p>"Then in that case, Angulimala, go to that woman and on arrival say to
  her, 'Sister, since I was born in the noble birth, I do not recall
  intentionally killing a living being. Through this truth may there be
  wellbeing for you, wellbeing for your fetus.'"</p>
  
  <p><a href="http://www.accesstoinsight.org/tipitaka/mn/mn.086.than.html">http://www.accesstoinsight.org/tipitaka/mn/mn.086.than.html</a></p>
</blockquote>

<p>The Buddha taught people guilty of the most heinous things to change their ways but to move on and not dwell on their previous errors. If the Buddha encouraged a man guilty of killing his father and a serial killer not to dwell on guilt, how much more would he encourage someone who committed adultery to simply have a change of heart and not dwell on guilt. </p>
