user19291: A question for the initial phase of starting meditation. 
To start meditation, it's important to know food first as per satipatthana. 
Materialistic food preserves cell-offspring reproduction inside body. In a way food helps in adultery. Whether we eat once a day or many times a day, no difference. 
Why to forcibly carry-on adultery within body at cellular level? Why not let the cells die naturally instead of allowing them to produce daughter cells by providing them required things. 
For this question, there is another subquery that can be considered, "what is natural?" "Is there anything as natural?" Various precepts might say adultery is natural, attraction of opp. identities(of any material) is natural but buddha gave a proper reason that can break this naturality. 
Is this merely a misguided term because natural is what it should be buddha's teachings but this teaching ends whole cycle and hence in the end, ends itself.
Where is natural (infinite process of causes & effects) in here? 
So subquery can be ,Why word natural has contradictory meanings as per above 2 paragraphs? 
EDIT 1:: Understood subquery part, only 1st remains now. 
Impermanence is natural and nature. Natural is not infinite rather it is universally acceptable.

user2282: 
What's the difference between eating food and adultery?

Big difference. Without food, you'll die and completely miss the rarest chance to cultivate the Path in this human form. You will not die due to not having adultery. And not only that you completely miss the rarest chance to cultivate the Path, you'd completely wreck it. Matter of fact, engaging in adultery and you're already "dead", and will continue to be dead countless times throughout this endless Samsara.

user471: Perhaps you are confusing the word "adultery" with the words "reproduction" and "sex".
When you eat food, you are definitely supplying energy to the body and giving it the necessary ingredients for cells to reproduce.
The cells in your body do not have sex to reproduce. Sex by the way, is not against the five precepts, if practised within the context of a committed exclusive monogamous relationship by lay persons.
Sex however, is not part of the monastic life, although food is. ChrisW's answer addresses this perfectly. Food is needed to sustain the body and you need to  consume food to survive, but sex is not needed for sustenance - hence a monk needs to eat food, but does not require sex.
The subject of food for the sustenance of a person seeking freedom from suffering, is well discussed in SN 12.63 (quoted below):

"And how is physical food to be regarded? Suppose a couple, husband &
  wife, taking meager provisions, were to travel through a desert. With
  them would be their only baby son, dear & appealing. Then the meager
  provisions of the couple going through the desert would be used up &
  depleted while there was still a stretch of the desert yet to be
  crossed. The thought would occur to them, 'Our meager provisions are
  used up & depleted while there is still a stretch of this desert yet
  to be crossed. What if we were to kill this only baby son of ours,
  dear & appealing, and make dried meat & jerky. That way — chewing on
  the flesh of our son — at least the two of us would make it through
  this desert. Otherwise, all three of us would perish.' So they would
  kill their only baby son, loved & endearing, and make dried meat &
  jerky. Chewing on the flesh of their son, they would make it through
  the desert. While eating the flesh of their only son, they would beat
  their breasts, [crying,] 'Where have you gone, our only baby son?
  Where have you gone, our only baby son?' Now what do you think, monks:
  Would that couple eat that food playfully or for intoxication, or for
  putting on bulk, or for beautification?"
"No, lord."
"Wouldn't they eat that food simply for the sake of making it through
  that desert?"
"Yes, lord."
"In the same way, I tell you, is the nutriment of physical food to be
  regarded. When physical food is comprehended, passion for the five
  strings of sensuality is comprehended. When passion for the five
  strings of sensuality is comprehended, there is no fetter bound by
  which a disciple of the noble ones would come back again to this
  world.

"Adultery" and "sex" does not mean the same thing either. Adultery is sex outside of a committed exclusive monogamous relationship, and this violates the precept on sexual (or sensual) misconduct.
What is natural and unnatural? Is food natural? Yes. Is sex natural? Yes.
Then doesn't the Buddha teach what is natural? Actually, the answer is no.
The idea of the self, craving, clinging, sensual pleasures, sexual attraction, passion of becoming, ignorance - all these are natural. But they lead to suffering and perpetuate suffering.
The Buddha realized after enlightenment that it's hard for him to teach people to go against their nature, which is what he said in SN 6.1 (quoted below):

"This Dhamma that I have attained is deep, hard to see, hard to
  realize, peaceful, refined, beyond the scope of conjecture, subtle,
  to-be-experienced by the wise. But this generation delights in
  attachment, is excited by attachment, enjoys attachment. For a
  generation delighting in attachment, excited by attachment, enjoying
  attachment, this/that conditionality and dependent co-arising are hard
  to see. This state, too, is hard to see: the resolution of all
  fabrications, the relinquishment of all acquisitions, the ending of
  craving; dispassion; cessation; Unbinding. And if I were to teach the
  Dhamma and if others would not understand me, that would be tiresome
  for me, troublesome for me."

However, in Iti 109 (quoted below), the Buddha indeed teaches man to swim against his nature to become free from suffering. Renunciation is against the flow i.e. it's not natural to man.

This was said by the Blessed One, said by the Arahant, so I have
  heard: "Suppose a man was being carried along by the flow of a river,
  lovely & alluring. And then another man with good eyesight, standing
  on the bank, on seeing him would say: 'My good man, even though you
  are being carried along by the flow of a river, lovely & alluring,
  further down from here is a pool with waves & whirlpools, with
  monsters & demons. On reaching that pool you will suffer death or
  death-like pain.' Then the first man, on hearing the words of the
  second man, would make an effort with his hands & feet to go against
  the flow.
"I have given you this simile to illustrate a meaning. The meaning is
  this: the flow of the river stands for craving. Lovely & alluring
  stands for the six internal sense-media. The pool further down stands
  for the five lower fetters. The waves stand for anger & distress.
  The whirlpools stand for the five strings of sensuality. The monsters
  & demons stand for the opposite sex. Against the flow stands for
  renunciation. Making an effort with hands & feet stands for the
  arousing of persistence. The man with good eyesight standing on the
  bank stands for the Tathagata, worthy & rightly self-awakened."


19291: <p>A question for the initial phase of starting meditation. </p>

<p>To start meditation, it's important to know food first as per satipatthana. </p>

<p>Materialistic food preserves cell-offspring reproduction inside body. In a way food helps in adultery. Whether we eat once a day or many times a day, no difference. </p>

<p>Why to forcibly carry-on adultery within body at cellular level? Why not let the cells die naturally instead of allowing them to produce daughter cells by providing them required things. </p>

<p>For this question, there is another subquery that can be considered, "what is natural?" "Is there anything as natural?" Various precepts might say adultery is natural, attraction of opp. identities(of any material) is natural but buddha gave a proper reason that can break this naturality. </p>

<p>Is this merely a misguided term because natural is what it should be buddha's teachings but this teaching ends whole cycle and hence in the end, ends itself.
Where is natural (infinite process of causes &amp; effects) in here? </p>

<p>So subquery can be ,Why word natural has contradictory meanings as per above 2 paragraphs? </p>

<p>EDIT 1:: Understood subquery part, only 1st remains now. </p>

<p>Impermanence is natural and nature. Natural is not infinite rather it is universally acceptable.</p>
 2282: <blockquote>
  <p>What's the difference between eating food and adultery?</p>
</blockquote>

<p>Big difference. Without food, you'll die and completely miss the <a href="https://accesstoinsight.org/tipitaka/sn/sn56/sn56.048.than.html" rel="nofollow noreferrer">rarest chance</a> to cultivate the Path in this human form. You will not die due to not having adultery. And not only that you completely miss the rarest chance to cultivate the Path, you'd completely wreck it. Matter of fact, engaging in adultery and you're already "dead", and will continue to be dead countless times throughout this endless <a href="http://www.palikanon.com/english/wtb/s_t/samsaara.htm" rel="nofollow noreferrer">Samsara</a>.</p>
 471: <p>Perhaps you are confusing the word "adultery" with the words "reproduction" and "sex".</p>

<p>When you eat food, you are definitely supplying energy to the body and giving it the necessary ingredients for cells to reproduce.</p>

<p>The cells in your body do not have sex to reproduce. Sex by the way, is not against the five precepts, if practised within the context of a committed exclusive monogamous relationship by lay persons.</p>

<p>Sex however, is not part of the monastic life, although food is. ChrisW's answer addresses this perfectly. Food is needed to sustain the body and you need to  consume food to survive, but sex is not needed for sustenance - hence a monk needs to eat food, but does not require sex.</p>

<p>The subject of food for the sustenance of a person seeking freedom from suffering, is well discussed in <a href="https://www.accesstoinsight.org/tipitaka/sn/sn12/sn12.063.than.html" rel="nofollow noreferrer">SN 12.63</a> (quoted below):</p>

<blockquote>
  <p>"And how is physical food to be regarded? Suppose a couple, husband &amp;
  wife, taking meager provisions, were to travel through a desert. With
  them would be their only baby son, dear &amp; appealing. Then the meager
  provisions of the couple going through the desert would be used up &amp;
  depleted while there was still a stretch of the desert yet to be
  crossed. The thought would occur to them, 'Our meager provisions are
  used up &amp; depleted while there is still a stretch of this desert yet
  to be crossed. What if we were to kill this only baby son of ours,
  dear &amp; appealing, and make dried meat &amp; jerky. That way — chewing on
  the flesh of our son — at least the two of us would make it through
  this desert. Otherwise, all three of us would perish.' So they would
  kill their only baby son, loved &amp; endearing, and make dried meat &amp;
  jerky. Chewing on the flesh of their son, they would make it through
  the desert. While eating the flesh of their only son, they would beat
  their breasts, [crying,] 'Where have you gone, our only baby son?
  Where have you gone, our only baby son?' Now what do you think, monks:
  Would that couple eat that food playfully or for intoxication, or for
  putting on bulk, or for beautification?"</p>
  
  <p>"No, lord."</p>
  
  <p>"Wouldn't they eat that food simply for the sake of making it through
  that desert?"</p>
  
  <p>"Yes, lord."</p>
  
  <p>"In the same way, I tell you, is the nutriment of physical food to be
  regarded. When physical food is comprehended, passion for the five
  strings of sensuality is comprehended. When passion for the five
  strings of sensuality is comprehended, there is no fetter bound by
  which a disciple of the noble ones would come back again to this
  world.</p>
</blockquote>

<p>"Adultery" and "sex" does not mean the same thing either. Adultery is sex outside of a committed exclusive monogamous relationship, and this violates the precept on sexual (or sensual) misconduct.</p>

<p>What is natural and unnatural? Is food natural? Yes. Is sex natural? Yes.</p>

<p>Then doesn't the Buddha teach what is natural? Actually, the answer is no.</p>

<p>The idea of the self, craving, clinging, sensual pleasures, sexual attraction, passion of becoming, ignorance - all these are natural. But they lead to suffering and perpetuate suffering.</p>

<p>The Buddha realized after enlightenment that it's hard for him to teach people to go against their nature, which is what he said in <a href="https://www.accesstoinsight.org/tipitaka/sn/sn06/sn06.001.than.html" rel="nofollow noreferrer">SN 6.1</a> (quoted below):</p>

<blockquote>
  <p>"This Dhamma that I have attained is deep, hard to see, hard to
  realize, peaceful, refined, beyond the scope of conjecture, subtle,
  to-be-experienced by the wise. But this generation delights in
  attachment, is excited by attachment, enjoys attachment. For a
  generation delighting in attachment, excited by attachment, enjoying
  attachment, this/that conditionality and dependent co-arising are hard
  to see. This state, too, is hard to see: the resolution of all
  fabrications, the relinquishment of all acquisitions, the ending of
  craving; dispassion; cessation; Unbinding. And if I were to teach the
  Dhamma and if others would not understand me, that would be tiresome
  for me, troublesome for me."</p>
</blockquote>

<p>However, in <a href="https://www.accesstoinsight.org/tipitaka/kn/iti/iti.4.100-112.than.html#iti-109" rel="nofollow noreferrer">Iti 109</a> (quoted below), the Buddha indeed teaches man to swim against his nature to become free from suffering. Renunciation is against the flow i.e. it's not natural to man.</p>

<blockquote>
  <p>This was said by the Blessed One, said by the Arahant, so I have
  heard: "Suppose a man was being carried along by the flow of a river,
  lovely &amp; alluring. And then another man with good eyesight, standing
  on the bank, on seeing him would say: 'My good man, even though you
  are being carried along by the flow of a river, lovely &amp; alluring,
  further down from here is a pool with waves &amp; whirlpools, with
  monsters &amp; demons. On reaching that pool you will suffer death or
  death-like pain.' Then the first man, on hearing the words of the
  second man, would make an effort with his hands &amp; feet to go against
  the flow.</p>
  
  <p>"I have given you this simile to illustrate a meaning. The meaning is
  this: <strong>the flow of the river stands for craving</strong>. Lovely &amp; alluring
  stands for the six internal sense-media. The pool further down stands
  for the five lower fetters. The waves stand for anger &amp; distress.
  The whirlpools stand for the five strings of sensuality. The monsters
  &amp; demons stand for the opposite sex. <strong>Against the flow stands for
  renunciation</strong>. Making an effort with hands &amp; feet stands for the
  arousing of persistence. The man with good eyesight standing on the
  bank stands for the Tathagata, worthy &amp; rightly self-awakened."</p>
</blockquote>
