However, if one is aware of one's sexuality beforehand, that doesn't really hold. I knew I was gay for at least 3 years before it was brought up in church or school, and a friend of mine in a similar situation figured out she was a lesbian a few months before it was brought up. In that sort of situation, all of the anti-gay rhetoric might convince one to try to act and/or become straight (as happened with my friend) or to research the issue and make sure one is actually gay in the face of all the crap one is going to get for it in future (as happened with me), but the most I could see that doing in terms of actually directly affecting sexuality in some way would be causing someone who's really bisexual to identify as purely gay in reaction ("You can't make me straight, I'm gonna ignore [opposite sex], so there!").
Also, if nurture really had that kind of effect, going through your formative years with people telling you gays are going to Hell, it's unnatural, etc. would be much more likely to cause you to turn out straight than one that says it's okay to be gay or that ignores the whole issue entirely. It's the old "Given how crappy gay people have it in many areas, why would anyone there choose to be gay?" argument: if you eat an apple every day after school and suddenly discover that it's an option to eat a candy bar instead, but people who eat a candy bar after school are shunned and hated and evil, then no matter how much your nature tells you that candy bars or delicious you're probably going to try pretty damn hard to convince yourself the apple is just dandy and you might eventually convince yourself that you truly do like the apple better...but you're going to have vastly fewer people go with the candy bar in that scenario than you would in a scenario where kids are asked "So, would you like an apple or candy bar?" or just "Whaddya want?" instead