[Rouge]

There definitely is dominance and submission. Think about it biologically; in doggy, the female (or reciever) is completely exposed. Neck, organs, spine, there's no protection. They're at a complete disadvantage, and being submissive is in their best interests (not that they wouldn't be having fun, but I imagine some encounters would have been hostile).

This also ties in with why some guys can have a very hard time with mmf threesums and/or are very possessive, biologically guys don't want to share "their" women. It made the chance of their genes passing on less likely if their mate(for lack of a better word. I'm tired, leave me alone! :P) was with other guys.

It's funny, but when you look at sex, you see that we are still a very primal species.