AIW for confronting my friend about flirting with my girlfriend?
So me (24M) and my girlfriend (23F) have been together for almost 2 years. I've known my friend Mark since college, we hang out pretty regularly, and he's always been around when my girlfriend and I do group stuff.
Last weekend we all went out for a friend's birthday. I stepped away to the bar for maybe 10 minutes, and when I came back I noticed Mark had his arm around the back of her chair, leaning in really close, laughing way harder than necessary at everything she said. When I sat back down he pulled away fast and got quiet.
Later that night I asked her about it and she said it was nothing and I was overreacting. The next day I texted Mark directly and told him it made me uncomfortable and to knock it off. He got defensive, said I was being paranoid and insecure, and that I was "ruining the friend group" by making it weird. A couple of our mutual friends have since said I overreacted and should've just let it go since nothing happened.
Now things are tense with Mark, my girlfriend thinks I embarrassed her, and I'm getting side-eyed by people in our group chat.
AIW for saying something instead of just letting it go?