I believe that you can raise a genetically straight boy to make him wear skirts (always in the color pink), give him only dolls to play with, and teach him all kinds of traditionally feminine mannerisms - and he's still going to come out straight. He'd going grow up and be attracted to and want to fuck females rather than males.
In an extreme and unhealthy scenario like that, I'd expect the kid to be confused and experience tremendous cognitive dissonance (
http://en.wikipedia.org/wiki/Cognitive_dissonance) during his developing years while under the heel of his insane parents, but once removed from the toxic environment and given free choice to dress and act the way he wants to, he'd be masculine.
I'm sure there are a small number of people who may be completely straight who pretend and flirt with the idea that they're gay, or extreme fetishists who may engage in homosexual behavior even if they're straight, but clearly the vast majority do not. Then you have bisexuals who apparently are attracted to both genders.
I think it's all hardcoded for us.