There's also the brief but very intense compilation of film scenes where women are raped and/or beaten up, which Kevin Smith states that he finds more offensive then depictions of consensual sex.
Values Dissonance: European ratings tend to be more permissive about sex and more restrictive about violence than American ratings, but are given no specific examples.
Outside of America, classification boards are usually administered by the government. Anything that doesn't fit in with the rating systems cannot be shown unless it's educational or sport-related. Incidentally, But I'm a Cheerleader got an M rating (equivalent to PG-13) in Australia, while American Beauty got an MA15+ (closer to the American R rating).
Though the MPAA promotes itself as a preferable alternative to government-mandated regulation/censorship of film, many believe MPAA ratings do carry the force of law. Imagine an unaccompanied small child trying to buy a ticket to an R or NC-17 film. Would any American theater allow this? The MPAA is a private organization with no law-enforcement authority. De jure, yes. De facto?