I believe that homosexuality arises from primate pack dynamics which are a subtle yet powerful but very poorly researched constituent of human psyche.

Sometime around puberty your personality gets baked in but along with that also your blueprint for behaviour within your social unit, heavily dependent on your relations with the other members of said unit. Most boys, having healthy relationships where they are allowed to hone their tendency for being dominant will naturally develop a heterosexual orientation. The outcasts, and especially the abused ones treated poorly are already established to have a very high chance of becoming homosexual. If you look from an evolutionary perspective not on the individual, as everyone has been doing for two hundred years, but instead on the societies humans form, suddenly you can start seeing the evolutionary advantages of segregating pack members early and automatically. Naturally, this pretty much works the same way in other animals. Bear in mind though that we are intellectually very far ahead of even primates and so in us this might have gotten further refined into a more permanent arrangement as time went on. Nature certainly must have seen benefit from this, though.

If you think of it as a kind of a filter it suddenly starts to make a lot of sense, especially with how men in prison invariably will form a homosexual underculture where those dominant will then leave prison and continue functioning heterosexually.