This transman who writes m/m fanfiction is obviously very interested in the latest turn the discussion has taken 😛I don't necessarily agree with everything, but more importantly, I'm surprised nobody's brought up one of the main problems with m/m fanfic: its very heavy heteronormativity.
I'm in a dead fandom, so basically I can do whatever I want, nobody's going to tell me off. But I have a friend who is in a slash sub-fandom of Harry Potter, and she regularly wails in private to me about how so many supposedly male/male fics are in fact male/female fics with the bottom guy written almost exactly like an uber-stereotypical girl: loving, empathetic, compassionate, listening, self-sacrificing, and of course, always giving in bed. In the meantime, the top character gets to be mean, violent, self-centered, manipulative, and always taking and taking, and the relationship is seen as romantic because the bottom "saves" and 'loves the top so much!"
So I would argue that yes, on the one hand, gay fanfic may wrongly encourage some girls to think of themselves as gay guys, but on the other hand, it also very much reinforces the worst stereotypes of straight relationships.
Even when it doesn't, I'd argue that most of the girls and women who write gay fanfic definitely don't think like gay guys and don't care one bit about actual gay sexuality; they are just exploring heterosexuality from a more remote, more detached point of view.
And then there's the omegaverse...