You sure as hell shouldn't feel good about it lilwing. That's just disrespectful to you and shows an extreme lack of caring on her part. You need to tell her how you feel about it, and that it really is bs for her to act that way. I really can't tell you much else...if I saw that happening w/my girl, my reaction would probably mirror what you're thinking you'd do if you saw it. Just keep in mind that after you attack the guy that's grinding w/your gf, you won't feel very good about it because you'll have just ended up making an ass of yourself, when otherwise she would come off looking like a complete *****. Best reaction to that, if she doesn't respect your feelings enough not to do it, is after you see it happen, you dump her, straight and simple.