Not necessarily. I can think of a couple movies off the top of my head that managed to show full frontal male nudity and still receive an R rating.
In society today, I think the main emphasis on any censorship is for children's sake and generally we fear the younger generation's sexual experimentation more than their mindless fighting. You go home with a black eye and a bloody nose, that's just kids rough-housing and taking it too far. You go home and tell your mom that you and a girl were touching each other in your "special places" and the response is "OH GOD! WHAT HAPPENED TO THE MORAL FABRIC OF OUR SOCIETY?!"
It's way harder to talk to kids about sex than violence, thus we fear media forcing the issue before we can prepare to give them "the talk". I'm sure some kind of warlike mentality comes into play somehow as well.