At the end of A.O. Scott's review of the new Marion Cotillard Oscar bait, "Rust and Bone," this is how the New York Times explains the French film's R rating.
"Rust and Bone” is rated R (Under 17 requires accompanying parent or adult guardian). Sex, fighting, killer whales and parental neglect.
If it deserves an R, the reason might really be this, from Scott's review:
A plot summary of “Rust and Bone” might suggest a maudlin, therapeutic fable of healing, and in many ways — or, let’s say, in hindsight — the movie is just that. But its conventional, sentimental essence (which is overtly revealed only at the end) is effectively complicated, indeed almost undone, by the jagged textures of the filmmaking and the naturalistic intensity of the performances. If this is a love story, it is also a combat picture, in which the principal characters are at war with themselves, each other and the cruelty of the world...
[Director Jacques] Audiard...sometimes seems impatient with the limitations of film as a visual medium. He wants you to smell the sweat and feel the fleshy impact of every moment, to climb inside the suffering, yearning skins of Ali and Stephanie and feel the things that they do, whether they are swimming, fighting or having sex.