I find it far more erotic watching same sex couples together than mixed, be it MM, FF, MMF, FFM etc. This is why I feel I'm bi-curious, I don't know if I'd enjoy it the same if I was part of the action.
There's just this intensity and understanding you pick up on watching same sex couples, they know what feels good and how to go about it because they're the same. You don't often see that kind of chemistry with MF couples the same way.