CLOV3R covered that pretty well. There's a huge difference between "when you become gay" and "when you noticed you were gay." There is a time in most people's lives when they think the opposite gender is gross and has cooties. Does that mean you're gay until X years old when you start becoming attracted to them? Not at all. It means your sexuality hasn't developed. Sexuality doesn't hit you like a brick wall either. You aren't just sitting there, and BAM, you want to go have sex with a woman. Whoever Kojinako is talking about just didn't happen to notice until they watched Hercules.

The evidence for prenatal hormones and their influence on sexuality is astounding. It's not even just one thing that somebody noticed. It's a huge combination of factors, all of which point to prenatal hormone exposure. Really, anybody that denies it is so biased... Well, I should say, homophobic. And if you're homophobic, that's a completely different topic that is huge lulz in itself, and of course you'll want it to be a choice.
And anyone who knows what causes homophobia will know why homophobes want sexuality to be a choice.