According to the MPAA's ratings guidelines, scenes containing nudity do not automatically qualify a movie for an R or NC-17 rating.
Under the guidelines for PG, it says (note: all emphasis in these quotes is mine):
There may be some profanity and some depictions of violence of brief nudity.
For PG-13, the guidelines state:
A PG-13 motion picture may go beyond the PG rating in theme, violence, nudity, sensuality, language, adult activities, or other elements, but does not reach the restricted R category.
More than brief nudity will require at least a PG-13 rating, but such nudity in a PG-13 rated motion picture generally will not be sexually oriented.
The R rating is where the "nature" of the nudity becomes a factor:
may include adult themes, adult activity, hard language, intense or persistent violence, sexually-oriented nudity, drug abuse or other elements.
So there is clearly a distinction in the MPAA system between "sexual" and "non-sexual" nudity, but there are no real guidelines on what those terms mean. Certainly topless women are not implicitly "sexual", or the guideline would make no sense. As an example, the movie Sheena was rated PG, but contains several brief topless and rear nudity. (Note that Sheena was assigned its rating before the addition of PG-13 in mid-1984).
What's less clear is if full frontal nudity would always qualify as "sexual" in nature, and if female vs. male nudity is a factor. (I know the MPAA now has a descriptor specifically for male nudity, but I don't know if it affects the rating.)
Has the MPAA ever assigned PG or PG-13 rating to a movie that included scenes of full frontal nudity?