On services like Xbox live and psn, it is about 90/10 male female for active players. I would call that male dominated.
Why? Not worth the shitstorm. I'll never really understand, I guess. Just can't empathize with the overwhelming need to harass, belittle, and attack people online just because of a single identifier. I'm glad I can't. I bet it feels really empty.
I haven't seen a study which addresses how many women do this, nor a clear breakdown on how many of them do it because they've experienced harassment or are just worried about it because of second-hand stories.