Nothing wrong mah. Don't be a hardassed jealous idiot. Just
because she's with you doesn't mean that she cannot have other guy
friends.
Even if you don't trust the guy, you should at the bare minimum
trust your GF. If your relationship cannot even have this simple
level of trust, how do you expect to progress further with
her.
I suggest that you do some soul searching.
Do remember that even if your GF really wants to fool around,
there's nothing you can do. Either grin and bear it, or move
on.