There are a whole lot of animal species that show sexual behavior that we could regard as non-heterosexual behavior, though. So it's probably unreasonable to assume that homosexuality can just die off by not being genetically inherited in some way. It also doesn't seem to support the idea of the upbringing of children being the cause.
I don't remember ever hearing about a connection of homosexuality and genetics inside a family either. So, hmm, maybe there are more fundamental genetic reasons why there is a chance to be homosexual within the animal kingdom?
It's not like I have studied this so I could be wrong. But no matter what the exact reason may be, we can probably say that while not being the norm, it's not socially constructed by humans.