You're a mean one, Mr. Grinch!
You really are a heel.
You're as cuddly as a cactus,
You're as charming as an eel,
Mister Griiiiinch!
You're a really bad banana
Meaning that even though you already have a girlfriend doesn't mean that you can't date someone else.
Bob: Dude look at those hotties man!
Mike: But I already have a girlfriend
Bob: Even though you're on a diet, doesn't mean that you can't look at the menu.
Mike: Oh OK then.
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!