For the same reason some choose to have sex with sheep.
I do not know whether homosexuality is determined by some genetic predisposition, or what. However, I do not think it is much of a choice either. If one states that homosexuality is a choice that would mean that anyone could choose to be homosexual. I could not be homosexual. I am not sexually attracted to other men, and I believ this is the consensus between most people. We really cannot choose who we are sexually attracted to, we just are. I don't know why I find certain women attractive and others not, I just do. I think this is what most people including homosexuals experience.