"Shirtless men are allowed in G-rated films, while topless women usually earn at least a PG-13; but naked men nearly always garner an R-rating (see: Sideways), while fully naked women are routinely seen in PG-13 movies (see: Titanic, rated PG-13). And you can forget about sex. Even non-graphic depictions of love-making will render an otherwise tame movie unacceptable for 16-year-olds (see: Never Let Me Go)".

Cited from: The Real Difference Between "PG-13" and "R" Rated Movies - NakedLaw by Avvo.com http://nakedlaw.avvo.com/bizarre/the-real-difference-between-pg-13-and-r-rated-movies.html#ixzz3Kd7RM63R