We all know the impact of humans on the planet, and that there are too many people that the biosphere can comfortably fit. Could the fact that some people are homosexual be a biproduct of this? Obliviously when you are attracted to the same sex you cannot reproduce and offspring (putting aside invitro, our most basic instinct wouldn't understand that.) The only way for a homosexualy couple to have children is to take in other peoples children which would not add to the already over populated Earth.
*This is not an anti-gay idea, merely a theroy as to why so many humans have this trait compared to other animals.