[Yosei]

Why not? Just because it hasn't been proven that being gay or not is genetically based, doesn't mean it isn't a possibility.

I reckon that whether an individual is gay or not may be based on either genetics or lifestyle influences. Things like this can be caused by a bit of both.

I agree. It could be something at birth that ultimately leads to your sexual orientation.
I know this is a weird story, but my sisters friend always wanted a daughter, but she ended up with a son. For the first 3 years of his life, she passed him off as a girl. Dressing him as a girl, and letting him grow his hair out.
He ended up gay.
I know its not exactly from birth, but it could help prove that it could be other things from way earlier on in life that influenced it.