Well, I'm bisexual, so I definitely don't see it as an either/or situation. I subscribe to the theory that sexuality exists within a continuum rather than as one of only three options/categories.
Ultimately, however, I think that behavior is a choice but attraction is not. People can choose to do all sorts of things, including things that are opposite their natural inclinations. Men and women who are all the way 100% super gay can choose to live in the closet and to pursue relationships with members of the opposite gender (and vice versa), but that's not the same thing as choosing to be straight. Or choosing to be gay. Even in straight relationships, though, people pretty frequently convince themselves to spend a lifetime with people they don't really love or to whom they're not really attracted, so it's not that huge a stretch of the imagination to suggest that someone could fake their way through a straight lifestyle without really being straight.
Yeah, someone might make the argument. It's a pretty ty one, though.