How about smoking or drinking alcohol? That is damn unnatural. So is driving a car, let alone flying.
Besides, humans aren't the only species with a certain share of homosexual specimen, which should prove it's not cultural.
Failing that, so far nobody has explained how "natural" or not has anything to do with morally right or wrong.
As for the OP. Since this thread ended being about the rights or wrongs of homosexuality instead of about marriage and what defines it, I'm afraid there is still a long way to go!