One reason may be simply because it feels good:
Not every sexual act has a reproductive function," said Janet Mann, a biologist at Georgetown University.
Clearly an activist scientist.
But she goes on:
"It could be a way that you strengthen bonds — that's one hypothesis," Mann told LiveScience. "Another is that it could be practice for heterosexual sex. Bottlenose dolphin calves mount each other a lot. That might benefit them later on."
Oh. So that's where the term "practicing homosexual" comes from.
But the one thing unique to humans with regards to homosexuality?
Homophobia.