You’re not overreacting. It was inappropriate for your boyfriend to grab your friend’s butt, and your feelings are reflective of that bad behavior. Your friend blurted out the accusation because she knew it was wrong, was uncomfortable and felt that making it public made her a non-party in the action and brought it to your attention and everyone else’s. She didn’t welcome the gesture or like it. Your boyfriend was either making a pass at her to test the waters and see if she responded to him (which she didn’t), or else he has impulse control issues that are going to be a big problem for you down the line.
You didn’t mention how old you all are or how long you’ve been dating, but my advice to you without knowing that information is to understand that you’ve brought it to his attention and I trust you’ve told him that you think it’s inappropriate and it makes you very uncomfortable on a whole bunch of levels. I hope he’s told you he won’t do it again. And now, I would advise you to let this go as one incident of bad boyfriend behavior that’s come to the surface, been discussed, and is now behind you.
If his behavior continues on this track, then you’ve got a bigger problem, but for now, let this just be a mistake he made that’s apologized for and the apology has been accepted.
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