@ Carpe diem;
Adding onto what battalions said, the way I personally differentiate between R and NC-17 is by thinking about what's generally shown during sex scenes in R-rated movies. There's usually passionate kissing, nudity,
sometimes oral sex, but you don't see the exact movements, details, and mechanics of it, you just
know they're having sex. I think, with R-rated sex, it's more about describing the sensations and emotions than the exact "insert x into y" details