The look on his face after you've wrapped him up in saran wrap and played with him for a long time, after you ask him, while holding a nice big sharp shiny knife, "Would you like me to cut your cock?"

Wait for his intake of breath. Watch his eyes.

Then add, "You know, out of the wrap so I can really fuck you. "

*grins*

It's even better when, despite the spikes of fear, that run through his body's nerves he says yes. He is trusting you with his manhood and his desire is raging enough to make him want to fuck so very desperately.

Then of course, running the dull side of the knife along his cock is lovely too, such lovely reactions. *sighs*