advice

So! A couple of nights ago my boyfriend had his friends over! All girls and some whom I'm really close with. I left the room to put our daughter to sleep and when I came back one of the girls were sitting on his bum, dry humping him ( she was wearing a dress so her coochie was touching him) my question is, am I over reacting? He said that because it's never the girl who goes behind when having sex it's ok and because he was just laying down he wasn't entertaining it, there shouldn't be an issue... I told him it comes down to respect and he said that I need to message the girl and tell her that it's disrespectful