I'm uncomfortable with my girlfriend going to see a band whose drummer is someone she previously had sex with before we were together
Do you have a problem with trusting her?
Yes, I just think that's improper. It's just wrong to me, especially since I find him extremely attractive, the idea of someone she had sex with looking at her body makes me feel jealous and uncomfortable.
I'm sorry you're feeling this way and you have all the right to. An ex is dangerous territory and she might still have feelings for him and is using the show as an excuse to have sex again. You need to stop her or tell her your feelings right now, because you don't want to regret being idle and possibly having your relationship ruined.
Okay I'll talk to her you don't understand what its like to think of someone in that way.
If that is something that really bothers you, then I support you in this decision. I think that it is good that you are standing up for what you really feel in this situation.