That's a very good question. I dont have a definitive answer to that and can only guess, but I think that many gay teenager discover their sexuality when they find themselves lusting after a male friend. Maybe it's because there aren't that many options when you're a gay boy in the closet, maybe it's the fact that guys sometimes see each other naked, masturbate together, talk about sex and stuff, and for a gay teenager it can develop into an attraction towards his friend. In most cases, though, the friend is straight and that attraction remains unfulfilled, it becomes a fantasy as the person grows into an adult.
Even as adults, gay guys are attracted to men, which largely includes straight guys. The ideal of male beauty that is promoted everywhere is the one of a straight guy, and gay guys are subjected to that just like everyone else. In a way, we are programmed or at least taught to find a certain set of characteristics attractive in a man, and one of them is being straight (or "acting straight"). The only way for a gay guy to fantasize about a straight guy is to imagine him "turning gay".
I think it has less to do with turning a straight guy gay and more to do with being attractive to guy you're attracted to, or finding a way to make your fantasy work.
Now, as to why those stories keep being claimed as true, that I really cannot answer.