I wonder why we feel so uncomfortable saying, "Don't do that, please." I know we don't wish to cause the other person discomfort, but that other person hasn't considered that before acting rudely, have they?

I encourage all of us who don't like the belly rubbing (me included) practise saying firmly but politely, "Don't do that, please." We don't owe anyone any explanation why we don't want our bodies touched. And if they have an issue with it, all you should need to say is, "I don't like it." That should be enough!