My suggested MPAA rating: R for pervasive strong language, sequences of boxing violence, and for a scene of sexuality.
Violence: 3/5 - Other than one scene towards the end, this doesn't get any more graphic than Rocky. In fact, Rocky may be a bit more graphic than much of this because a) it's in color and b) it has that scene where Rocky's eyelid gets ripped off with a razor.
Sex: 3.5/5 - Hard to miss when it comes up. There's one sex scene that lasts about four minutes; I skipped over it and missed not one bit of the plot. There are also sexual references.
Language: 5/5 - VERY strong, and very offensive. "F--k" is used at least 70 times throughout the film, "c--t" is used two or three times near the very end of the film, and there are instances of other swear words such as "c--k" and "s--t".