No way someone's butt, crotch, or other privates are going to be in my face or near. I would just offer a polite, "excuse me, you're crowding me" and that should do it.
You know what else I really hate? When I've sat down on a plane, and there are still people filing by, stowing away their luggage in the bins. When they do it in the row that you're sitting in, and their butts, crotches, or stomachs are right smack there in your face then. Yuck!!