What if a character says "Oh, go fuck yourself"?
Arnold Schwarzenegger used that line in
The Sixth Day. It was PG13.
Anyway, this is my impression of MPAA ratings over the years:
Late 1960's
G - Most mainstream movies
M - Edgy movies
R - Very edgy movies
X - Even edgier movies
Early 1980's (before PG13)
G - Kids' movies
PG - Most mainstream movies
R - Horror movies, Sex comedies, art movies, hard-core action movies
X - Porn
Today
G - Animated kids' movies
PG - Live action kids' movies
PG13 - Most mainstream movies, including stuff as mild as
The Devil Wears Prada and some of the Harry Potter films, as well as stuff as brutally violent as
Taken, and everything in between. Almost all summer action movies go here.
R - Slasher movies, sex comedies, art movies
NC17 - Really edgy art movies