I won't tell if you don't

Convenient phrase used to assure any person being approached by an interested party that they made the right decision in initially turning them down them with the "sorry, I have a boyfriend/girlfriend/significant other of non-specific gender" line. Often followed by a cheeky wink, just to really bring it home.

The war-cry of the asshole.

Guy: "hey gorgeous, do you think I could take you out some time?"
Girl: "aw that's sweet, and I'm flattered, but I have a boyfriend"
Guy: "no sweat baby, I have a wife and three kids at home. I won't tell if you don't"
Girl: "oh gosh really? Thank goodness, that was my main concern. Pick me up at 7, and bring a three-pack of Durex"