answer:I don’t think it’s been even remotely proven or dis-proven. It’s a big, complicated question, and rather than try to simplify it, I would just say start reading. Biology and sexual orientation- Wikipedia I’m straight and my brother is gay, and this question makes me realize, that I have never given a moment’s thought to “why”. It’s merely one fact about him – he’s about six feet tall, gay, has dark hair, he likes dogs, etc. I wouldn’t wonder why he’s gay any more than I would try to figure out why he’s right-handed. I’m not criticizing the question, I am just adding a small personal anecdote.