But encouraging homosexuality goes against our instinct to breed.
People have sex with each other for lots of other reasons. Sex for procreation is only one drive that humans manage. If you consider how infertile males and females actually are and the amounts of sex acts that go on between actual fertilization, I think it could be safely said that sex for procreation is not necessarily even one of the primary reasons for sex.
Dr. Christopher Ryan wrote a book called Sex at Dawn that goes into the evolutionary history of sex in humans and other primates. Very fascinating reading. According to this book, the sexual experience is a bonding experience and is not limited to sex between males and females. Regarding orientation, people in other cultures have had a much freer view of who could bond with who. Which brings up a point about sexual orientation that I think this study points at. If people in other cultures can regularly take same sex lovers, then some part of orientation is obviously cultural phenomenon.