I am surprised this film is not getting an NC-17 and suprised by the fact that there is no protest to force the film to be re-rated at NC-17.
If the film is rated NC-17 it removes it from most retail outlets.
It also keeps the film out of most public schools as a "diversity training film".
A lot of people, including me, agree with you. The "mystery" is the ratings board, no doubt about it...last I read about this issue, this film's odd bypass of the NC-17, was that the ratings system itself was denigrated and thus, this film got a pass (conveniently, is the point, the ratings system was called into question where this film was/is concerned, the system alleged to be nonsensical and not meaningful, and so, hey, it doesn't apply to this film!). That's my summation of an article I read in VARIETY not too long ago, trade paper for the film and television industry.
I can't recall WHO opined thusly, just that the ratings question, as you raise it and others have, was discussed in VARIETY and part of the article included the discussion that suggested the rating system was nonsensical...and thus...bypassed.
It's arche manipulation, no doubt about it!