Well, Molly sometimes tries to hump King when he's asleep, she knows that he will not allow her to even do a single "pump" before knocking her off.
What he does to her sometimes is an entirely different thing. He DOES have sex with her! She doesen't like it one bit. I try to make him stop it, but usually I just see him pumping away out in the back yard, with her trying to turn her head around and nail King, and it's too late. I took them to the vets a few weeks ago, and the vet saw him doing it to her while we were in the waiting room. A friend had their leashes while I went to the bathroom. The vet thought it was amusing more than anything else, but was surprised that he was able to do it, as he's been neutered for over 3 years now. A couple of times, he seems to have come close to tieing with her, but just can't quite do it. He does "finish" though. She is totally confused as to what he's doing, but he knows exactly what he's doing to her.
Most of the time though, it's just a dominace thing. He is extremely dominant, but not aggressive.