A magical chastity belt
One day, as King Arthur mulled over the maps in his war room, he realized his current crusade would take him far out of the way of Camelot for quite a good many months. Fearing for Guinevere's safety by herself in a castle full of men for that amount of time, he requested of Merlin that some form of device be forged for her protection.
Finishing the task quickly, Merlin presented to his highness a magical chastity belt with a small hole in the underside to allow the Queen to relieve herself, and a very tiny guillotine which slid cleanly across the opening. With his worries put to rest, Arthur made haste to his crusades, leaving a small number of the Knights behind.
Upon his return, he called them all into a room and addressed them: "My lads," he said "I have nothing to ask of you, except that you drop your pants." Every single one of them did as they were told, with a look of pained shame on their faces, revealing that each one of them was missing a part of his manhood. All except for Lancelot.
The king embraced him immediately saying "Lance, Lance my dearest friend, I cannot respect you enough for your chastity. What have you to say for yourself?" Lancelot gave him a shocked expression, but proceeded slowly to say: "Mmf, mah ruuuh woaaah mhffhm oahhham arh!"