Film makers are usually puzzled as to what criteria drive the MPAA to assign an NC-17. That's quite difficult to figure out indeed.
American Beauty, straight guy masturbates naked in the shower - rated R.
But I'm A Cheerleader, gay girl masturbates fully-clothed in her bedroom - rated NC-17. When originally submitted to the Motion Picture Association of America rating board, But I'm a Cheerleader received an NC-17 rating. In order to get a commercially-viable R rating, Babbit removed a two second shot of Graham's hand sweeping Megan's clothed body, a camera pan up Megan's body when she is masturbating, and a comment that Megan "ate Graham out" (slang for cunnilingus). American Pie (also released in 1999), which features a teenage boy masturbating, was given an R rating.
Single White Female, straight guy has oral sex performed on him - rated R.
Boys Don't Cry, gay girl has oral sex performed on her - rated NC-17. Initially assigned an NC-17 rating from the MPAA, the content was strongly toned down for the US release, where the film was rated R. Peirce displayed anger over the fact the MPAA wanted the sex scene between Brandon and Lana removed but were satisfied with the overall brutality and violence in the murder scene.
Where The Truth Lies, gay girl has oral sex performed on her - rated NC-17. The film received an NC-17 rating in the United States due to scenes depicting a threesome and graphic lesbian sex. Egoyan condemned the MPAA decision as "a violent act of censorship", while Bacon stated, "I don't get it, when I see films (that) are extremely violent, extremely objectable sometimes in terms of the roles that women play, slide by with an R, no problem, because the people happen to have more clothes on." Both suggested that homophobia may have played a role in the decision, as the film deals in part with repressed homosexuality.
Mysterious Skin, gay guy is shown having sex with another man from behind - rated NC-17.
Unfaithful, straight guy is shown having sex with a woman from behind - rated R.
Sideways, straight sex scenes - rated R.
Henry & June, gay sex - rated NC-17. Henry & June was the first film to receive the MPAA's rating of NC-17, which had been devised as a replacement for the X rating. NC-17 was intended to signify serious, non-pornographic films with more violence or (especially) sexual content than would qualify for an R rating. The inclusion of the postcard Nin views at the start of the film (which is of Hokusai's The Dream of the Fisherman's Wife), and some scenes of le Bal des Beaux Arts contributed to the NC-17 rating.
American Psycho, rated NC-17 because of a three way sex scene, but not for the axe and chainsaw murders. The Motion Picture Association of America (MPAA) gave the film an NC-17 rating for a scene featuring Bateman having a threesome with two prostitutes. The producers excised approximately 18 seconds of footage to obtain an R-rated version of the film.
Sin City, full of dismemberments and decapitations and grizzly forms of violence, rated R.
The Dreamers, sex among three young people, rated NC-17. The film was controversial in the United States because of its nudity and sexual content. There are two versions: an uncut NC-17-rated version, and an R-rated version that is about three minutes shorter.
The Cooler, shows a woman's pubic hair, rated NC-17. Tthe MPAA originally rated the film NC-17 because of a glimpse of Maria Bello's pubic hair during a sex scene. An edited version rated R was released in theaters. A director's cut has been broadcast by the Independent Film Channel and Cinemax.
Basic Instinct is rated R for "strong violence and sensuality, and for drug use and language". It was initially given an NC-17 rating by the MPAA, but under pressure from TriStar, Verhoeven cut 35–40 seconds to gain an R rating. Verhoeven described the changes in a March 1992 article in The New York Times: Actually, I didn't have to cut many things, but I replaced things from different angles, made it a little more elliptical, a bit less direct. The film was subsequently re-released in its uncut format on video and later on DVD.
More about the MPAA here:
Biggest MPAA Ratings Controversies - Business Insider
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