One thing that is interesting about the Vinaya rules—and by that I mean the original Vinaya, not the later texts under discussion here—is that they rather frequently impose weaker proscriptions on same-sex activity than on hetero sexuality.
The following examples are off the top of my head, so I may get some details wrong! May @brahmali correct me!)
For example, if a monk were to engage in sexually suggestive and lewd conversation with a woman, it is a serious offence of sanghadisesa, which imposes a suspension of privileges and a period of public penance. If he does so with a man, whether lay or another monk, it is only a minor offence of wrong-doing (dukkata), which is cleared with a simple confession.
Similarly, if a monk touches a woman with sexual intent—groping, kissing, etc.—then it is a sanghadisesa, but if it is another man, it is a dukkata.
When it comes to penetrative sexual intercourse, this is an even more serious offence—a parajika, entailing immediate and permanent expulsion from the Sangha. However, sexual penetration is defined in such a way that it must involve a penis, so same-sex activity among nuns can never incur such a penalty.
There are a number of similar examples, but none of the opposite sort. That is to say, there are no cases where same-sex activity is penalized to a greater degree than hetero activity.
So, this is the legal situation in Vinaya.
Now, when it comes to Vinaya, there is very loose relation between the legal penalty of something and its moral gravity. That is to say, we can’t argue that because something has a lesser Vinaya penalty that it is therefore less morally serious. Still, there is something noteworthy about this pattern of events which calls for an explanation. None is given in the texts themselves, so we are left to our own hypotheses.
It seems to me that there are two possible reasons for this situation.
One possibility is that these cases are basically meaningless, and they arose purely for formalistic and historical reasons. The rules are originally laid down in a case where hetero sexual activity is the issue. Later, additional cases were added, and the additional cases tend to be derived applications of the rule that incur a lesser penalty. Such situations, where the nature of the rules is determined by formalistic evolution, are extremely common in the Vinaya.
The second explanation assumes that the differences are intentional: the Vinaya deliberately reduces the penalty for same-sex activity in the Sangha. Why might it do that? I think is it is because it is so much harder to keep such rules strictly if they apply to your comrades and brothers, who you live with all the time. A carelessly crude joke here, or a stray touch there, and a monk could easily fall into doubt as to whether they are guilty of a serious offence.
I don’t think either of these are necessarily complete explanations. Still, taken together I think they may help understand why the situation is the way it is.
While the reasons for the situation are debatable, what is not up for debate is the rules themselves. And it seems that, so far as the Vinaya is concerned, not only is there no question that gay people can and should lead the holy life, there is no question as to their capacity to do so in a monastic community of their own gender.
Not only that, but the Vinaya actually relaxes several important rules, the effect of which is to let gay people get by in situations where they might otherwise struggle. Whether it was the original intent or not, the Vinaya assumes that a lighter set of rules is enough, and gay people can manage their sexuality responsibly.