I don't think that someone's born being gay. Choice might've been the wrong word ... It is just something you find out about yourself. It is the same with everything else. To state a rather stupid example: In early childhood I detested cheese, I really hated it. However taste is changing, isn't it? So nowadays, I like cheese a lot. Not every kind of cheese to be honest but still, I like it. (:
And I think it's about the same with being gay/straight or whatever. It is just something you find out. Like taste. I don't think that someone is born hating cheese. At least I do not want to think so. That way, life would be pretty boring, without being able to discover the things you like/dislike.
I heard that many people change their sexual orientation even in later life ... They simply discover that they like the other sex more/too. It just depends when and if you really realize that.