In short, they hold a double standard. It's ok for them to make a straight character go gay for a person, but it's not ok for a gay character to go straight for a person. I can't take that line of thought seriously.
Although I'm seriously wearing on this topic (see the SJW thread in the Lobby), I'll just go ahead and take a quick stab at this one. When you have a story about a previously self-identified straight character going gay for someone, it ties in to some ideas that I think a lot of people can relate to, whether straight or gay (for instance, the pressure to hide one's true desires for fear of social ostracism). This doesn't happen with the reverse storyline: It's extremely rare (at best) for straight people to pretend to be gay because they think that doing so will allow them to avoid social ostracism, so a storyline about a gay person going straight will generally not have the same kind of thematic resonance on that level.
There's also the long history of viewing homosexuality as a kind of disease or disorder that needs to be 'cured,' and stories about a previously gay person falling in love with someone of the opposite gender have a way of playing into these ideas; I don't want to say that such a story can never be told, but you have to seriously consider why you want to tell this kind of story, what kind of message you want to get across, and what kinds of unintentional messages you might be sending. We never experience media in a vacuum; our cultural baggage always affects the way in which we experience pretty much anything, let alone games or movies.
Anyways, I'm pretty sure I've hit my post quota (see what I did there?) on this kind of subject matter, so I'll check out and let others get this thread back on topic.