I think it's genetic, or as birdgirl said when your extremely young.
I don't think your up bringing has very much to do with it...although I suppose it could effect homophobia and denying your (true) sexuality.

I consider myself bisexual, leaning more towards the gay side of things. But I had early experiences with both male and female (and continued to up untill about 6 months ago) and I don't believe that had anything to do with my current sexuality.

I like men...lol thats it...in the same way straight people are attracted to the opposite sex, I am attracted to the same. Nothing I can do about it, I wouldn't want to do anything about it...I am completly comfortable and happy with it.

Edit: I did however grow up with 2 older sisters, and I got on extremely well with girls from a very young age. But I also had male friends from an early age...don't think that effected anything either.