I absolutely don’t think women can’t write m/m or gay fiction (please re-read that as many times as you need to believe me, okay?) I also don’t see anything wrong with using a pseudonym. I don’t even think there’s anything inherently off about using a pseudonym that isn’t aligned with the gender you identify with. But if you’re adopting a whole persona and identity from a culture – be it a race, sexual orientation, gender, neurotype or any other group to which you don’t belong – and you choose to speak from that role, you’re stepping over a line.