My feeling about the whole male nudity vs female nudity thing is that the male genitalia are simply humorous.
WHY don't you see a lot of male nudity in movies? Well...think about when you DO see male nudity in movies...No matter what is happening in the scene, it instantly seems to make the whole thing funny. For example, the shower room fight scene in "Eastern Promises". Here this man is fighting for his life, but all the audience (in my theater anyway) seemed to notice was Viggo's little fella thwacking around and the giggles were non-stop.
Monty Python recognized the ridiculousness of the male accessory and made it, in many cases, an inspiration for silliness.
http://www.youtube.com/watch?v=nGRPFUYUUdQ
