Because the man is most often stronger, and he is the one who penetrates. Even the act of penetration is some of aggression. A sexually aggressive women with violent sexual fantasies will never be looked at with as much fear or disgust as a man, for the reasons I mention. A woman can't rape a man, nor does a man have the fear of an unwanted pregnancy through being a rape victim.

Watch violent porn all you like. My opinion is all porn is bad, and not for religious reasons. It desensitizes the penis and gives unrealistic expectations of what real sex is like.

EDIT: Before someone calls me on it. I'm not downplaying a man being a rape victim by another man. I understand that would be as equally as degrading as a woman being a rape victim.