I have never heard of any other animal in the animal kingdom who have sex with the same gender. Male birds don't have sex with other male birds, female tigers don't have sex with other female tigers. That's b/c it is not normal, wholesome, good behavior. It's not natural that's why. The procreation of the species depends on the natural sexual intercourse between the opposite sexes.
Humans with all our intelligence do things b/c we can, even if it's wrong. We have free will, but we also can get corrupted by it, if we don't have moral standards and protocol.