Definitely psychological. I'm in a long-distance relationship (although this is the last night of that -- we're moving in together tomorrow!), and some of the kinkiest moments between us have been over the phone. He likes to talk dirty to me when he knows I'm in public -- or worse, with my parents. Somehow, he always knows how to make me very very quiet...

Too much reliance on physical force makes someone seem like they *aren't* really in control, IMHO -- especially when they seem really angry, it's like their emotions are in control of them. To me, that is not as much fun. I like to feel like my s/o is in control and I don't have to worry about anything.