user5380: Is it true that the Buddha didn't allow women in the Sangha? If this is true then why is that? 

user4742: Some people "blame" Ven.Ananda for initiating the Bhikkhuni Sasana.  However, if the Buddha did not want to establish the Bhikkhuni Sasana, no matter how much anyone pleaded he would have said no. Yet the Buddha did establish it, not Ven. Ananda, although Ven. Ananda was essential for this. 
Ven. Ananda absorbed the brunt of this drastic social change the Buddha created. Ven. Ananda deflected the criticism from the Buddha; He voiced the valid reasons why women should be allowed to be ordained,  instead of the Buddha having to voice them by his own initiative. Imagine had the Buddha initiated the Bhikkhuni order unprompted like he did the Bhikkhu order? Ven. Ananda was the catalyst in an event, that I think was going to happen anyway. 
Finding out why the Buddha was initially hesitant to establish the Bhikkhuni order is academic now. Anyone who intentionally discourages women or hinders them thinking that Bhikkhuni are bad for the sasana in my opinion goes against the Buddha. What matters is how can we collectively as the four fold sasana attain a sure footing on the path.
Whether the path will last in the future or not depends not on whether women are allowed to be ordained or not, but whether men and women strive to learn the dhamma, ponder upon it, memorize it, practice it themselves  and teach others. 

"For the Blessed One, O Lord, spoke these words to me: 'I shall not
  come to my final passing away, Evil One, until my Bhikkhunis have come
  to be true disciples — wise, well disciplined, apt and learned,
  preservers of the Dhamma, living according to the Dhamma, abiding by
  the appropriate conduct, and having learned the Master's word, are
  able to expound it, preach it, proclaim it, establish it, reveal it,
  explain it in detail, and make it clear; until, when adverse opinions
  arise, they shall be able to refute them thoroughly and well, and to
  preach this convincing and liberating Dhamma.'
  -- Mahaparinibbana Sutta DN 16


user-1: I think it's important to understand that the Buddha was very much a product of his culture.  It's a bit silly to judge him by current social mores and standards.  Initially, the Buddha believed that women were too important to the household to allow for their ordination.  I'm fairly sure that there were other reasons as well - not the least of which was the danger women might face wandering alone around ancient India, the sexual "threat" they might be to other monks, etc.     
There are a couple of quotes from the Vinaya cited in these to articles if you care to read more - 

Female Monks In Buddhism
Focus on Full Ordination for Buddhist Women


user254: I don't know what its reason is for saying this, but this footnote says,

It seems that the Buddha did not absolutely refuse Maha-Pajapati Gotami, but perhaps wished to test her determination. It would have been a very difficult thing for aristocratic ladies in those days to do — to become nuns and live a hard life in the forest, subsisting on almsfood. Ladies then had no experience in looking after themselves or organizing anything, as their social scope under Brahmanism was very much restricted.

Note that alhough it's true to say (i.e. it agrees with the suttas to say) that the Buddha "didn't originally allow", it's not true to say that "he didn't allow",

Ananda saw the Buddha's step-mother with swollen feet, covered with dust, eyes full of tears at the gate of the monastery of Vesali. When he asked her compassionately for the reason of her sorrow, she replied that the Master had three times rejected her request for the establishment of an order of nuns.
Thereupon Ananda rephrased his request [...] Ananda here brought two arguments to bear. [...] In response to these arguments the Buddha agreed to the establishment of an order of nuns, provided certain cautions and rules were followed.


user715: As far as I can tell, the Buddha didn't really offer clear reasons for his reluctance to ordain women, yet being the Buddha, he must have given it a lot of consideration. He didn't think women were less capable, this much he makes clear, but he must have had his reasons.

“Lord Buddha, can women attain enlightenment?” The Buddha said to
  him, “Ananda, yes of course they can.” He said, “If they can, why
  don’t you allow them to join the Sangha, learning and practicing
  directly, especially your foster mother who looked after you when you
  were young.” In fact, the Buddha at first denied Gotami the chance to
  ordain so as to make them sure that this is what they wanted to do.

source: Female monks in Buddhism, Bhikkhuni Dhammcaro
There are obvious practical reasons one can guess,

safety of the women in the forests, 
safety of the monks and nuns from temptations, mental pollutants and false accusations, 
safety of the order from corruption and schisms over the issue, 
safety of the laity.

The Buddha makes clear in the vinaya that he would like to make the true Dharma last as long as possible this time round, unlike previous times when some Buddhas didn't regulate the order. This may have also played on his mind.

After the disappearance of these Buddhas, the Blessed Ones, and their
  immediate disciples (buddhanubuddha), those last disciples of various
  names, of various clans, of various births, who had gone forth from
  various families, the holy life too disappeared. It is as if,
  Sāriputta, various flowers lying loose on a piece of wood are
  scattered about and blown about by the wind. (V 3:8)

There are some statements attributed to the Buddha that the admission of women in the order would shorten the lifespan of the Dhamma. However, this isn't universally accepted as having been said by the Buddha. Anyway, the dharma is now 2500 years old, so it hasn't come to pass.

If, Ananda, women had not obtained the going forth, the holy life,
  Ananda, would have lasted long, the True Teaching would have endured
  for a thousand years. But, Ananda, since women have gone forth, the
  holy life now would not last, the True Dharma would endure only for
  500 years. 10  (Vinaya Cullavagga 10.1.6/V 2:255; A 4:278; Miln
  4.1.55-61)    

source: Dharma ending age, Piya Tan
Finally, we must learn to accept what has come to pass. Neither can we ever truly understand the responsibilities and complexities the Buddha had to balance to spread the dharma, nor can we rely on historical accounts to be accurate. There are historians (Williams, Liz 2000 'A Whisper in the Silence: Nuns before Maha-paja-patı-?' Buddhist Studies Review) who have spent time trying to conjecture that there were nuns before Mahapajapati Gotami, which is well and good for a historian, but a waste of time for a practitioner.

At one time the Blessed One was staying at Kosambii in Simsapaa Grove. 
  Then the Blessed One, taking a few Simsapaa leaves in his
  hand, said to the monks: "What do you think, monks? Which are the more
  numerous, the few leaves I have here in my hand, or those up in the
  trees of the grove?"
"Lord, the Blessed One is holding only a few leaves: those up in the
  trees are far more numerous."
"In the same way, monks, there are many more things that I have found
  out, but not revealed to you. What I have revealed to you is only a
  little. And why, monks, have I not revealed it?
"Because, monks, it is not related to the goal, it is not fundamental
  to the holy life, does not conduce to disenchantment, dispassion,
  cessation, tranquillity, higher knowledge, enlightenment or Nibbaana.
  That is why I have not revealed it.

source: Simsappa Sutta

user143: Interesting, this is the first question answered in this month's Buddhism.SE Book-of-the-Month Women in Buddhism - Questions & Answers.

It was the right time for Maha Pajapati (the Buddha's step mother) to consider following
  the teaching and the practice of the Buddha seriously. But
  when she approached and asked for permission the Buddha
  simply said, “Please do not ask so.” The Tripitaka, which
  is the most important primary source, did not provide any
  reason for not allowing women to join the Order.

Maha Pajapati and 500 other women shaved their heads, donned yellow robes, and walked to where the Buddha was. Ananda went to the Buddha on their behalf to ask for their permission to join the sangha, but the Buddha's response was "“Ananda, please do not ask so.”.
Some of the considerations and concerns that might have been present include the fact that:

According to
  Indian social mores, to lead a religious life is not the path
  for women. ibid

There may also have been practical concerns as these women had lived in comfort before and the reality of living a reclusive life outdoors might be difficult. The current sangha did not have enough teachers who would have been suitable to teach this large group of women. There could have been criticism that having women join the sangha was breaking up families. These may have been some of the reasons behind the Buddha's hesitation to allow women to join the sangha.
When Ananda asked the Buddha if women were capable of attaining spiritual enlightenment; the Buddha's answer was yes. The book notes:

We have to mark this statement, as this is the first time
  in the history of religion that a religious leader declared
  openly that men and women are equal on spiritual grounds.
  Previously in the Hindu context, the Vedas, the most sacred
  religious texts, were available only to men. Buddhism has
  transcended race, nation, caste and gender differences to 
  declare that the highest spiritual achievement transcends
  obstacles or discrimination of gender. With this important
  reason, the Buddha allowed women to join his Order. ibid

So while women weren't allowed to join the sangha immediately, eventually they were. 

5380: <p>Is it true that the Buddha didn't allow women in the Sangha? If this is true then why is that? </p>
 4742: <p>Some people "blame" Ven.Ananda for initiating the Bhikkhuni Sasana.  However, if the Buddha did not want to establish the Bhikkhuni Sasana, no matter how much anyone pleaded he would have said no. Yet the Buddha did establish it, not Ven. Ananda, although Ven. Ananda was essential for this. </p>

<p>Ven. Ananda absorbed the brunt of this drastic social change the Buddha created. Ven. Ananda deflected the criticism from the Buddha; He voiced the valid reasons why women should be allowed to be ordained,  instead of the Buddha having to voice them by his own initiative. Imagine had the Buddha initiated the Bhikkhuni order unprompted like he did the Bhikkhu order? Ven. Ananda was the catalyst in an event, that I think was going to happen anyway. </p>

<p>Finding out why the Buddha was initially hesitant to establish the Bhikkhuni order is academic now. Anyone who intentionally discourages women or hinders them thinking that Bhikkhuni are bad for the sasana in my opinion goes against the Buddha. What matters is how can we collectively as the four fold sasana attain a sure footing on the path.</p>

<p>Whether the path will last in the future or not depends not on whether women are allowed to be ordained or not, but whether men and women strive to learn the dhamma, ponder upon it, memorize it, practice it themselves  and teach others. </p>

<blockquote>
  <p>"For the Blessed One, O Lord, spoke these words to me: 'I shall not
  come to my final passing away, Evil One, until my Bhikkhunis have come
  to be true disciples — wise, well disciplined, apt and learned,
  preservers of the Dhamma, living according to the Dhamma, abiding by
  the appropriate conduct, and having learned the Master's word, are
  able to expound it, preach it, proclaim it, establish it, reveal it,
  explain it in detail, and make it clear; until, when adverse opinions
  arise, they shall be able to refute them thoroughly and well, and to
  preach this convincing and liberating Dhamma.'
  -- Mahaparinibbana Sutta DN 16</p>
</blockquote>
 -1: <p>I think it's important to understand that the Buddha was very much a product of his culture.  It's a bit silly to judge him by current social mores and standards.  Initially, the Buddha believed that women were too important to the household to allow for their ordination.  I'm fairly sure that there were other reasons as well - not the least of which was the danger women might face wandering alone around ancient India, the sexual "threat" they might be to other monks, etc.     </p>

<p>There are a couple of quotes from the Vinaya cited in these to articles if you care to read more - </p>

<ul>
<li><a href="http://www.buddhapadipa.org/buddhism/female-monks-in-buddhism/" rel="nofollow">Female Monks In Buddhism</a></li>
<li><a href="http://fpmt.org/mandala/archives/older/mandala-issues-for-1988/april/focus-on-full-ordination-for-buddhist-women/" rel="nofollow">Focus on Full Ordination for Buddhist Women</a></li>
</ul>
 254: <p>I don't know what its reason is for saying this, but <a href="http://www.accesstoinsight.org/lib/authors/hecker/wheel273.html#fn-29" rel="nofollow noreferrer">this footnote</a> says,</p>
<blockquote>
<p>It seems that the Buddha did not absolutely refuse Maha-Pajapati Gotami, but perhaps wished to test her determination. It would have been a very difficult thing for aristocratic ladies in those days to do — to become nuns and live a hard life in the forest, subsisting on almsfood. Ladies then had no experience in looking after themselves or organizing anything, as their social scope under Brahmanism was very much restricted.</p>
</blockquote>
<p>Note that alhough it's true to say (i.e. it agrees with the suttas to say) that the Buddha &quot;didn't originally allow&quot;, <a href="http://www.accesstoinsight.org/lib/authors/hecker/wheel273.html#section-5" rel="nofollow noreferrer">it's not true to say that &quot;he didn't allow&quot;</a>,</p>
<blockquote>
<p>Ananda saw the Buddha's step-mother with swollen feet, covered with dust, eyes full of tears at the gate of the monastery of Vesali. When he asked her compassionately for the reason of her sorrow, she replied that the Master had three times rejected her request for the establishment of an order of nuns.</p>
<p>Thereupon Ananda rephrased his request [...] Ananda here brought two arguments to bear. [...] In response to these arguments the Buddha agreed to the establishment of an order of nuns, provided certain cautions and rules were followed.</p>
</blockquote>
 715: <p>As far as I can tell, the Buddha didn't really offer clear reasons for his reluctance to ordain women, yet being the Buddha, he must have given it a lot of consideration. He didn't think women were less capable, this much he makes clear, but he must have had his reasons.</p>

<blockquote>
  <p>“Lord Buddha, can women attain enlightenment?” The Buddha said to
  him, “Ananda, yes of course they can.” He said, “If they can, why
  don’t you allow them to join the Sangha, learning and practicing
  directly, especially your foster mother who looked after you when you
  were young.” In fact, the Buddha at first denied Gotami the chance to
  ordain so as to make them sure that this is what they wanted to do.</p>
</blockquote>

<p><strong>source</strong>: <a href="http://www.buddhapadipa.org/buddhism/female-monks-in-buddhism/" rel="nofollow noreferrer">Female monks in Buddhism, Bhikkhuni Dhammcaro</a></p>

<p>There are obvious practical reasons one can guess,</p>

<ul>
<li>safety of the women in the forests, </li>
<li>safety of the monks and nuns from temptations, mental pollutants and false accusations, </li>
<li>safety of the order from corruption and schisms over the issue, </li>
<li>safety of the laity.</li>
</ul>

<p>The Buddha makes clear in the vinaya that he would like to make the true Dharma last as long as possible this time round, unlike previous times when some Buddhas didn't regulate the order. This may have also played on his mind.</p>

<blockquote>
  <p>After the disappearance of these Buddhas, the Blessed Ones, and their
  immediate disciples (buddhanubuddha), those last disciples of various
  names, of various clans, of various births, who had gone forth from
  various families, the holy life too disappeared. It is as if,
  Sāriputta, various flowers lying loose on a piece of wood are
  scattered about and blown about by the wind. (V 3:8)</p>
</blockquote>

<p>There are some statements attributed to the Buddha that the admission of women in the order would shorten the lifespan of the Dhamma. However, this isn't universally accepted as having been said by the Buddha. Anyway, the dharma is now 2500 years old, so it hasn't come to pass.</p>

<blockquote>
  <p>If, Ananda, women had not obtained the going forth, the holy life,
  Ananda, would have lasted long, the True Teaching would have endured
  for a thousand years. But, Ananda, since women have gone forth, the
  holy life now would not last, the True Dharma would endure only for
  500 years. 10  (Vinaya Cullavagga 10.1.6/V 2:255; A 4:278; Miln
  4.1.55-61)    </p>
</blockquote>

<p><strong>source</strong>: <a href="http://www.themindingcentre.org/dharmafarer/wp-content/uploads/2009/12/1.10-Dharma-ending-age-piya-proto.pdf" rel="nofollow noreferrer">Dharma ending age, Piya Tan</a></p>

<p>Finally, we must learn to accept what has come to pass. Neither can we ever truly understand the responsibilities and complexities the Buddha had to balance to spread the dharma, nor can we rely on historical accounts to be accurate. There are historians (<em>Williams, Liz 2000 'A Whisper in the Silence: Nuns before Maha-paja-patı-?' Buddhist Studies Review</em>) who have spent time trying to conjecture that there were nuns before Mahapajapati Gotami, which is well and good for a historian, but a waste of time for a practitioner.</p>

<blockquote>
  <p>At one time the Blessed One was staying at Kosambii in Simsapaa Grove. 
  Then the Blessed One, taking a few Simsapaa leaves in his
  hand, said to the monks: "What do you think, monks? Which are the more
  numerous, the few leaves I have here in my hand, or those up in the
  trees of the grove?"</p>
  
  <p>"Lord, the Blessed One is holding only a few leaves: those up in the
  trees are far more numerous."</p>
  
  <p>"In the same way, monks, there are many more things that I have found
  out, but not revealed to you. What I have revealed to you is only a
  little. And why, monks, have I not revealed it?</p>
  
  <p>"Because, monks, it is not related to the goal, it is not fundamental
  to the holy life, does not conduce to disenchantment, dispassion,
  cessation, tranquillity, higher knowledge, enlightenment or Nibbaana.
  That is why I have not revealed it.</p>
</blockquote>

<p><strong>source</strong>: <a href="http://www.accesstoinsight.org/tipitaka/sn/sn56/sn56.031.wlsh.html" rel="nofollow noreferrer">Simsappa Sutta</a></p>
 143: <p>Interesting, this is the first question answered in this month's Buddhism.SE Book-of-the-Month <a href="http://www.buddhanet.net/pdf_file/qanda-women.pdf">Women in Buddhism - Questions &amp; Answers</a>.</p>

<blockquote>
  <p>It was the right time for Maha Pajapati (the Buddha's step mother) to consider following
  the teaching and the practice of the Buddha seriously. But
  when she approached and asked for permission the Buddha
  simply said, “Please do not ask so.” The Tripitaka, which
  is the most important primary source, did not provide any
  reason for not allowing women to join the Order.</p>
</blockquote>

<p>Maha Pajapati and 500 other women shaved their heads, donned yellow robes, and walked to where the Buddha was. Ananda went to the Buddha on their behalf to ask for their permission to join the sangha, but the Buddha's response was "“Ananda, please do not ask so.”.</p>

<p>Some of the considerations and concerns that might have been present include the fact that:</p>

<blockquote>
  <p>According to
  Indian social mores, to lead a religious life is not the path
  for women. ibid</p>
</blockquote>

<p>There may also have been practical concerns as these women had lived in comfort before and the reality of living a reclusive life outdoors might be difficult. The current sangha did not have enough teachers who would have been suitable to teach this large group of women. There could have been criticism that having women join the sangha was breaking up families. These may have been some of the reasons behind the Buddha's hesitation to allow women to join the sangha.</p>

<p>When Ananda asked the Buddha if women were capable of attaining spiritual enlightenment; the Buddha's answer was yes. The book notes:</p>

<blockquote>
  <p>We have to mark this statement, as this is the first time
  in the history of religion that a religious leader declared
  openly that men and women are equal on spiritual grounds.
  Previously in the Hindu context, the Vedas, the most sacred
  religious texts, were available only to men. Buddhism has
  transcended race, nation, caste and gender differences to 
  declare that the highest spiritual achievement transcends
  obstacles or discrimination of gender. With this important
  reason, the Buddha allowed women to join his Order. ibid</p>
</blockquote>

<p>So while women weren't allowed to join the sangha immediately, eventually they were. </p>
