Unfaithful

[Rondy]

Suspecting her husband of infidelity, the woman attempted to put an end to it by arousing his jealousy.
"What would you say if I told you that I've been sleeping with your best friend?" she asked provocatively.

"Well," he mused, "I'd have to say that you're a lesbian!"

[McMouse]

The answer I heard was “bad dog”

[Spicerack]

^ That's disgusting and better.