Groping someone is done for the purpose of sexual arousal. You can grope someone by their exposed legs and according to you that wouldn't be sexual assault because the legs are exposed in public. There is nothing special about the butt either. If you are in the middle of attacking someone and you go from punching their face to trying to dig your nails into their butt when they turn around and try to get away, no prosecutor is going to tack on a sexual assault charge just because you touched a part of their body that can't be exposed in public.

Clearly what I said isn't a description of what prosecutors actually do but a statement about the perversion of language. But in your defense of the language you have made some ridiculous statements about what the actual prosecutorial standards are.