I actually had the chance to do this when some guy turned up to date my younger niece, when they were both 17. He was alone with me for a few minutes, and he turned to me and laughed and said "Thank god her dad's not here, or it'd be time for the third degree, huh?"
I smiled and said "That'd be my job. Now I'm not gonna threaten you or pretend I've got a gun or anything, kid, but do you have any younger family you love?"
"Yeah. I got a baby sister, she's 14."
"And what would you do if some deadbeat douchebag came and took her on a date, and fucked her?"
He puffed up, all manly "I'd fucking KILL him!"
"Right," I replied. "Then you know how I feel."
He brought her back home at an early time, and reports were he was a total gentleman all night. They're still dating three years later and he never so much as kissed her for the first several months.