style - How to talk about certain anatomy without sounding vulgar or cowardly?

Let's talk about butts, because butts are safe, but really this question is for other parts of the anatomy as well.

There are 100s of words for butt. They seem to fall into one of a few categories. There are the cowardly words like "gluteus maximus", and there are vulgar words like ass. You can also try to avoid the whole thing by saying something like "he moves his hands below his waist". This, to me at least, when I read it also sounds cowardly. When I read it from other authors my mind always goes "Just say the word we are all adults here!"

My book deals with many heavy ideas like torture and rape. The characters curse all the time, and the characters can throw the word "ass" around all the time, but any time it is part of the narration it immediately sounds bad, and I get bad feedback from my test readers.

What is the best approach to not sound vulgar, and at the same time not undermine the maturity of the writing? I am asking specifically about narration, not about character speech.