Oh Dean. That was the wrong thing to say. Lucifer snaps the threatened gag into existence, squatting down to shift his grip on Dean's hair to his jaw to force his mouth open for the gag. Once it's in, it's quickly fastened in place to keep Dean's mouth open for as long as Lucifer wants it to be. "Just remember, you asked for this. All of it."

Then he's standing again and gripping Dean's hair again, dragging his head forwards as he guides his cock into Dean's mouth with his other hand. He thrusts into Dean's mouth as soon as he's in. If Dean wasn't going to fulfill his side of things, Lucifer will just have to take what he wants by fucking his mouth.