user-1: Namo Buddhaya! 
(I am a Theravada Buddhist)
The Vinaya says monks & nuns cannot be together in private (it would be an offense). 

But what if someone is a homosexual, how would they go about
  ordaining, as they would not be able to technically be with men then (similar to nuns)?

I would have a hard time believing the Buddha would discriminate against someone who is a homosexual. 
IF they cannot ordain, 

what is the reason that they can't ordain as according to the Buddha? ( WITH SOURCE)

The Buddha chose to ordain woman, and homosexuals for sure can overcome their sexual urges just as a woman can towards men AS WELL, so why can't homosexuals ordain?
(Please, if possible, cite sources!)
Metta! 

user295: Homosexuals may fall under asittaka pandaka. They cannot ordain.

asittakapandaka - A man who gains satisfaction from performing oral sex on another man and from ingesting his semen, and only becomes sexually aroused after ingesting another man's semen.

Source: Pandaka

user254: I don't want to spend time on this topic, but fyi ...
A partial answer from Wikipedia Buddhism and sexual orientation -- Paṇḍakas

The paṇḍaka is a complex category that is variously defined in different Buddhist texts. In the earliest texts, the word seems to refer to a socially stigmatized class of trans-feminine and/or cross-dressing people, some of whom may have been sex workers. Paisarn Likhitpreechakul argue that these people are grouped together as special types of those suffering impotence due to seminal absence/deficiency rather than gender roles.
Paṇḍaka is categorized along with other stigmatized groups who are excluded from ordination as well; those with physical disabilities such as deafness or dwarfism, or those who have committed crimes. "The Story of the Prohibition of the Ordination of Pandaka" from the Vinaya claims that the ban is a response to the example of a paṇḍaka monk with a desire to have sex. Being refused by other monks, he had sex with animal handlers, who then told the wider community and brought disgrace upon the sangha.

So it's from Vinaya commentary, apparently, if you like to research that.
So that's why topics like What is a pandaka? revolve around wonering what the word "paṇḍaka" even means.
"Are you a paṇḍaka?" isn't one of among the questions asked at ordination, so I don't know at what point in the process the question even arises.

This overview of the Vinaya says on page 100,

The twelve equivalent p›r›jikas include the eleven disqualified types who should
  not be ordained as bhikkhus in the first place. If they happen to be ordained,
  their ordination is invalid; once they are found out they must be expelled for life
  (Mv.I.61-68; see BMC2, Chapter 14 for details). They are—

a pa˚˜aka (essentially, a eunuch or a person born neuter—see Saºgh›disesa
  2),
a “non-human” being, (this includes n›gas, petas, devas, and yakkhas),
a hermaphrodite,
a person who poses as a bhikkhu without having been ordained,
a bhikkhu who has ordained in another religion without first giving up his
  status as a bhikkhu,
a person who has murdered his father,
a person who has murdered his mother,
a person who has murdered an arahant,
a person who has sexually molested a bhikkhunı,
a person who has maliciously injured a Buddha to the point of causing him to
  bleed, and
a person who has dishonestly caused a schism in the Saºgha, knowing or
  suspecting that his position was contrary to the Dhamma-Vinaya.

These eleven equivalent p›r›jikas apply to bhikkhunıs as well.
  The twelfth equivalent p›r›jika, which applies only to bhikkhunıs, is the case
  where a bhikkhunı leaves the Bhikkhunı Saºgha and takes up the role of a lay
  woman (Cv.X.26.1). Unlike the bhikkhus, the bhikkhunıs have no formal
  procedure for disrobing. If they leave the Saºgha, they are not allowed to
  reordain for the rest of this lifetime.

A "parajika" rule is a rule for which the consequence is disrobing i.e. no longer being a member of the sangha.

-1: <p><em>Namo Buddhaya! 
(I am a Theravada Buddhist)</em></p>

<p>The Vinaya says monks &amp; nuns cannot be together in private (it would be an offense). </p>

<blockquote>
  <p>But what if someone is a homosexual, <strong>how would they go about
  ordaining</strong>, as they would not be able to technically be with men then (similar to nuns)?</p>
</blockquote>

<p>I would have a hard time believing the Buddha would discriminate against someone who is a homosexual. </p>

<p><strong>IF</strong> they cannot ordain, </p>

<blockquote>
  <p>what is the <strong>reason</strong> that they can't ordain as according to the Buddha? ( WITH SOURCE)</p>
</blockquote>

<p>The Buddha chose to ordain woman, and homosexuals for sure can overcome their sexual urges just as a woman can towards men AS WELL, so why can't homosexuals ordain?</p>

<p>(Please, if possible, cite sources!)</p>

<p>Metta! </p>
 295: <p>Homosexuals may fall under asittaka pandaka. They cannot ordain.</p>

<blockquote>
  <p>asittakapandaka - A man who gains satisfaction from performing oral sex on another man and from ingesting his semen, and only becomes sexually aroused after ingesting another man's semen.</p>
</blockquote>

<p>Source: <a href="https://en.wikipedia.org/wiki/Pandaka" rel="nofollow noreferrer">Pandaka</a></p>
 254: <p>I don't want to spend time on this topic, but fyi ...</p>

<p>A partial answer from Wikipedia <a href="https://en.wikipedia.org/wiki/Buddhism_and_sexual_orientation#Pa%E1%B9%87%E1%B8%8Dakas" rel="nofollow noreferrer">Buddhism and sexual orientation -- Paṇḍakas</a></p>

<blockquote>
  <p>The paṇḍaka is a complex category that is variously defined in different Buddhist texts. In the earliest texts, the word seems to refer to a socially stigmatized class of trans-feminine and/or cross-dressing people, some of whom may have been sex workers. Paisarn Likhitpreechakul argue that these people are grouped together as special types of those suffering impotence due to seminal absence/deficiency rather than gender roles.</p>
  
  <p>Paṇḍaka is categorized along with other stigmatized groups who are excluded from ordination as well; those with physical disabilities such as deafness or dwarfism, or those who have committed crimes. "The Story of the Prohibition of the Ordination of Pandaka" from the Vinaya claims that the ban is a response to the example of a paṇḍaka monk with a desire to have sex. Being refused by other monks, he had sex with animal handlers, who then told the wider community and brought disgrace upon the sangha.</p>
</blockquote>

<p>So it's from Vinaya commentary, apparently, if you like to research that.</p>

<p>So that's why topics like <a href="https://buddhism.stackexchange.com/q/11207/254">What is a pandaka?</a> revolve around wonering what the word "paṇḍaka" even means.</p>

<p>"Are you a paṇḍaka?" isn't one of among the <a href="http://www.buddhanet.net/e-learning/buddhistworld/ordination1.htm" rel="nofollow noreferrer">questions asked</a> at ordination, so I don't know at what point in the process the question even arises.</p>

<hr>

<p><a href="https://www.accesstoinsight.org/lib/authors/thanissaro/bmc1.pdf" rel="nofollow noreferrer">This overview</a> of the Vinaya says on page 100,</p>

<blockquote>
  <p>The twelve equivalent p›r›jikas include the eleven disqualified types who should
  not be ordained as bhikkhus in the first place. If they happen to be ordained,
  their ordination is invalid; once they are found out they must be expelled for life
  (Mv.I.61-68; see BMC2, Chapter 14 for details). They are—</p>
  
  <ul>
  <li>a pa˚˜aka (essentially, a eunuch or a person born neuter—see Saºgh›disesa
  2),</li>
  <li>a “non-human” being, (this includes n›gas, petas, devas, and yakkhas),</li>
  <li>a hermaphrodite,</li>
  <li>a person who poses as a bhikkhu without having been ordained,</li>
  <li>a bhikkhu who has ordained in another religion without first giving up his
  status as a bhikkhu,</li>
  <li>a person who has murdered his father,</li>
  <li>a person who has murdered his mother,</li>
  <li>a person who has murdered an arahant,</li>
  <li>a person who has sexually molested a bhikkhunı,</li>
  <li>a person who has maliciously injured a Buddha to the point of causing him to
  bleed, and</li>
  <li>a person who has dishonestly caused a schism in the Saºgha, knowing or
  suspecting that his position was contrary to the Dhamma-Vinaya.</li>
  </ul>
  
  <p>These eleven equivalent p›r›jikas apply to bhikkhunıs as well.
  The twelfth equivalent p›r›jika, which applies only to bhikkhunıs, is the case
  where a bhikkhunı leaves the Bhikkhunı Saºgha and takes up the role of a lay
  woman (Cv.X.26.1). Unlike the bhikkhus, the bhikkhunıs have no formal
  procedure for disrobing. If they leave the Saºgha, they are not allowed to
  reordain for the rest of this lifetime.</p>
</blockquote>

<p>A "parajika" rule is a rule for which the consequence is disrobing i.e. no longer being a member of the sangha.</p>
