UPDATE: 40 minutes after I posted this, the Motion Picture Association overturned Blue Valentine's NC-17 designation and re-rated it R.
Critics of American movie ratings long have puzzled over the system that gives an R (under age 17 not admitted without parent or guardian) to a movie in which a woman is carved up by a chain saw and an NC-17 to one that shows a woman being sexually pleasured.
From such ratings by the Motion Picture Association of America, one might conclude that sexual violence against women is OK for American teenagers to see, but that they must be 18 to see consensual, loving sex.
The most recent example of this peculiar standard is the MPAA's NC-17 rating of Blue Valentine, Derek Cianfrance's heartbreaking portrait of a foundering marriage starring Ryan Gosling and Michelle Williams, which shows a discreet scene (in medium-longshot) of Gosling pleasuring Williams while both are clothed.
As Steven Zeitchik notes in his excellent piece in the L.A. Times, what makes Valentine's rating even more baffling is that the recent (and comparably discreet) girl-on-girl scene of Mila Kunis pleasuring Natalie Portman in Black Swan received an R rating from the MPAA. One explanation for this disparity is that the MPAA, an industry group financed by and representing the major studios, tends to be more flexible with studio films (Black Swan is a Fox release) than it is with independent productions (Blue Valentine is distributed by The Weinstein Company).