whatever exists in nature is natural , so there is nothing as "abnormal" which exists, nature eliminates watever is unfit by natural selection, and human sexuality is too complex to understand, so homosexuality is a nature's way of course to keep a ctrl on human population, another trick of nature just like the introduction of HIV.
overpopulation leads to mass extinction of the species, so it's one of the ways to keep human species under ctrl.
http://www.actionbioscience.org/newfrontiers/eldredge2.html
Overpopulation and overspecialization leads to mass extinction of species . Human species is going to face it soon :
The 6th mass extinction crisis.