I'd like to point out that not all nudity is the same. The problem is most people (at least, in this country...United States) can't tell the difference between sex and nudity and often treat them as one of the same. Same goes for the MPAA, which is why most films with nudity in them are usually rated R, regardless of what other content they contain.
If it were up to me, nudity without sex would be rated G while nudity with sex would keep its R rating, as usual.