The US is a presumed heterosexual society. The presumption is that we are all going to be straight: our parents, family, friends, acquaintences assume as much from the day that we’re born. We therefore all start as straight; that’s why you don’t hear of anyone choosing to be straight, because for all intents and purposes, we’re all “born” straight. When environmental forces (conscious or not) change our socialized preference, that’s when people become/choose homosexuality, bisexuality, etc....
I think it's accurate to say we're born straight, as biology certainly attests to being born straight as well as other areas of science and environmental forces.
My profile has a lot of information on the subject.