Some people's do. Whether it's a matter of becoming ready to follow that path, (As is, I'm sure, often the case) or whether it's a chemical thing is really still open to debate. To say "No, your orientation cannot change. You were always that way and just couldn't accept it," is virtually the same as saying, "There's no such thing as a bi-sexual. They're all just gays who can't accept it." It may sometimes be true, but to say it's always true is rather presumptuous, IMO.