Actually, at least three of the authors mentioned elsewhere in this discussion are fanfic writers gone pro (If you count Lackey, four. And one of the people I was thinking of in my count of trans characters is, too. That's just the ones where I happen to know for sure - I wouldn't be surprised at all if a few of the others were.)

I certainly agree with you that the m/m publishing genre is not the same as books from any genre that feature gay characters, but there are a fair number of "slash writers gone pro" who wind up in SF, too.