There is a sorta theory about the possibilities of there being several genes controlling homosexuality. Basically if you have some of these genes you may become a more caring father etc etc, and hence have more offspring (the reason they havent just been bred out) however if you have a certain number, and maybe raised in a certain way then you become homosexual.
Personally i can see the merits of this theory, but i reckon theres more to it than this.
As far as it being a response to overpopulation, even if it there was a way the human body monitored population levels, surely a simple single gene with no other function would literally wipe itself out first time it was expressed, and be no use as a continuous method of control?
Give a man a fire and he's warm for a day. Set a man on fire and he's warm for the rest of his life....