A related thing is true about the use of sex in films. It is very, very difficult to make nudity and sex work in films (and the theatre!) -- the immediate response (let me check out her/his body parts) and the intimacy pulls the viewer out of the scene. These kinds of things are almost always a failure that the movie then has to recuperate from to reassociate the viewer with the plot. That is one reason why filmed sex scenes (outside of pornography) are so poorly filmed -- we have all seen endless shots of arms and legs and bellies and all you think is: this is as far as they were allowed to go; or, when will we move on to the next scene; or, isn't the music here junk, and so on. There are very, very few sex scenes that are an integral part of a movie -- the only one I can think of, offhand, is Donald Sutherland and Julie Christie in Don't Look Now -- and only because of the infinite sadness of it. I would be interested in any other examples that people can think of.