Why is it that if one happens to be watching a movie with a sex scene they'll graphically show the full naked body of a woman, but almost never of the man?

It’s like showing boobs and pubic hair and everything else on is woman are OK, but showing a penis is not. We accidentally landed on a stupid show once, lots of sex. The women were prancing about the room, fully naked, while the men were talking while demurely tucked under the bed sheets.

Part of this question was prompted by the ball bells ad question. It’s OK to show bouncing boobs, but it’s not OK to show bouncing balls?

What is up with that?