Boyfriend going to concert

I’m not a big kpop fan, I don’t know much about it but I do know that their fans can sometimes be a bit too obsessive sometimes. My boyfriends two friends seem to really like kpop, they send photos of them in their group chat (only girl groups) and have profile pictures of kpop girls. My boyfriend likes listening to kpop, which I think nothing bad of , of course. The thing is those friends invited him to a concert before and he declined because he didnt like the group. I told him I wouldnt be sure if Id be comfortable with him going and he got upset at me and didnt understand why. Now its kind of stuck in my head and I’m unsure if he might go someday or not, even though he told me chances are low he will go to their concert. I dont know if im being unreasonable here I will never force him not to do something but I think it would make me very uncomfortable especially if he goes with those friends. I dont know how to change my perspective here, those girls I know go through diets, use makeup and other procedures to look the way they do, and theyre all beautiful.