I don't want to hang out with my girlfriend
Why? Do you have a reason why you won't want to spend time with her?
I don't want to miss any opportunities to meet new girls
It's understandable to want to meet new people, but its wrong to try to meet someone new when you already have a girlfriend. You should break up with someone before you end up cheating on them. Otherwise you're going to hurt them due to your own selfishness.
You are being too naive, nothing happens if I hang out with someone else or my girlfriend.
I am sure it is okay if it is just meeting people for fun and not anything sexual or romantic as that is considered cheating. Before you meet anyone though, try to tell your girlfriend about it so that you are sure she's okay with it.