Meaning that if you already have a girlfriend/lover, doesn't mean that you can't check out other girls.
Guy #1: Hey look at those chicks over there!
Guy #2: Buddy, I already have a girlfriend.
Guy #1: Just because you're on a diet, doesn't mean that you can't look at the menu.
Guy #2: I suppose you're right. *Gazes at girls for 5 minutes* Damn! This is like Baywatch!