The excuse you should ALWAYS give to your girlfriend when she bitches to you about grabbing/slapping her ass.
Zach: (slowly reaches for a game of grab-ass)
Kim: What the fuck, Zach! We're in public!
Zach: Wha...WHAT!?!? That wasn't me! That was The Underpants Monster!!!!
Kim: Oh, I'm sorry for yelling.
Zach: Damn right you are! (slaps Kim's ass)