Argument with girlfriends daughter?

No, you are real MAN for not retaliating. Your girlfriend may have gotten mad if you would've done what you know best to him. Thumbs up for being a man in the situation, THEN still being able to sit down and talk to all of them. Teenage girls are so, so difficult to keep a relationship with. They already think they don't have to listen to you, or anyone else for that matter. You should also discuss with her Mom, the levels of respect that she should have for you, you being in the position your are in. The best thing you can do is talk to her and keep the relationship between you two as kosher as possible. As far as the 21 year old, he really didn't want that either, because he was there available, ready and willing to talk it out.