A lot of movies show male genitals, but the scene is always ment to be funny, and make they crowd feel so awkward they have to laugh.
I think they'll show a boob and not a penis, because there are a whole bunch of men who want to see some "boobage".
But NO ONE wants to see a penis. Woman don't want to see, and men definitely don't want to see it. You know?
The only time you'll see one, is if it's in a movie like: Forgetting Sarah Marshal, Dewy Cox, or Will Ferrell's comedy show where he's George Bush (One Last Night With George Bush:You're Welcome America)
Well, anyway I hope I helped answer your question.
There are just a whole bunch of perverted men out there, and there aren't any perverted woman out there oggling up some dude scene in any of the movies I just listed. LOL :)