I suppose their thought by that time was "there's no girl/man, let's have sexual intercouse with anyone". It doesn't even mean they were bi-sexual once you can't sure know they had really attraction by one of the same gender.I'm still reading this thread, but thought I may as well post now. I'm straight, but have nothing wrong with gays, bi-sexuals, pansexuals, asexuals or any other kind of sexuality. I have had bi-curious thoughts, but I've never met a guy that made think that I want to... you get the picture.
As for the choice v birth argument, I disagree with both sides. I think that you are affected by how you are raised. I mean take the ancient Greek, they clearly thought of homosexuality as perfectly usual. If you were raised in a place where bisexuality was the norm, chances are you would be bisexual. If you were raised in a town of homosexuals (mind you, actually being born in the first place would be difficult) you would be far more likely to be gay.
I don't mean to offend anyone by comparing, but that's like a dog that, lacking a female, copulate another male dog.