I'd honestly say it's more nuture, than nature.

But then again, there's that weird statistic that says "For every boy child you have, there's a third more chance that the next boy child is homosexual."

I don't know why... but it might be something to do with being around lads so much that you start to feel attracted to them. There's definitely something happening like that in all Boys/Girls schools - quite a few people who go to single-sex schools turn out homosexual.