There are those who propose that it is population-pressure driven.
We know rats get agressive it the population of the cage is too large. I think people do this too. I don't see how having lots of people (like in big cities) leads to homosexuality, birth control is available and lots of sex can be had. Homosexual men have lots of sex, because each has a partner with the sex drive of a male. Thus they are both males. but attracted not to females. I am not referring here to prison homosexuality, (where women are not available, that is another thing, and is not the kind of homosexuality we are talking about here.) But I would not propose that population pressure has anything to do with it other than high population will allow a larger base number of homosexuals (at the same percentage).