my boyfriend and his girl friends.

okay, so my boyfriend always hangs out with a girl alone. i feel really uncomfortable, and sometimes we even argue about it. he thinks that i'm being too clingy and annoying. i feel really insecure about it because i feel that my boyfriend is gonna leave me sooner or later. so i tried to join him and his girl friend for an outing but he doesn't allow me to join them. i feel that there is something on between them and idk if i should do anything about it. would you allow your boyfriend hanging out with a girl alone?