I write a lot of M/M... but I've done my research, and I'm not talking about renting videos either. I have a very good friend I've known since we were 13 or so. He's gay, proud of it, and is my main source of information about "how does it feel, what do you think about, are men actually like that, is that even physically possible"? Just because I'm not a man doesn't mean I can't write about it. If that was the case then when I write Het I wouldn't be able to have the man interacting with the woman - just kinda standing there like a cardboard cut out.
Writing is a patchwork quilt of all kinds of emotions, experiences, feelings and well - work. I've found that research is key. There are a few things I won't write about... yet. My own emotional wellbeing dictates that I don't write about spousal abuse, rape, slavery or anything else along those lines. One of these days though it will probably be some form of therapy - who knows?
But the big thing here is what do you feel comfortable writing? Everyone has their comfort zone, and I for one am quite happy that they do. If you're uncomfortable writing M/M or F/F or scat or whatever it's fine by me. To each his or her own I say. It's your writing and no one can force you to write what you don't want to. I even went so far as to fail an essay test in a writing course I took because the subject I had pulled out of the hat was loathsome to me and I couldn't pick again. I just walked out of the class. Damn professors and their God like complexes.