Well since I've seen bulls trying to mount other bulls I would have to say it's most likely genetic (actually homosexuality has been seen in most higher order mammals - and don't ask me for links, fuck off and look it up for yourself). I can't really see someone saying 'hey, I want to be the target of discrimination and hate crimes' or 'hey, I want to have fewer rights than everyone else in a society that's supposed to protect everyone under the same constitution'.
I'm straight and I'm quite sure no one taught me how to get a boner looking at a naked girl. I suppose the nurture part of the nature/nurture arguement comes from men not knowing how to react to the fact that two guys want to go at it. It's understandable since sexuality seems to be hardwired and anything different is met with fear. I say 'understandable', but definitely not reasonable.
