Must we really turn this thread into a discussion of biology vs preference? Or can we just agree that, like plenty of other things, it's an interesting idea for a porn game, and leave it at that.
Besides, while I think it's probably impossible to change someone's sexuality (at least without crazy stuff like brainwashing, magic etc.), I don't deem it implausible that there are bisexual people who are not yet aware that they are bisexual, which is really all that's necessary to make this scenario realistic.