Merlin assures the king that anything that is put through the hole in the chastity belt will be immediately cut off with magic.
Satisfied, King Arthur goes on his hunt. When he returns several days later, he immediately goes to the Knights of the Round Table and asks all of them to drop their pants.
Incredibly, all of their dicks are cut off, except Sir Lancelot. King Arthur, furious, yells at his knights, “See! Look how none of you are trustworthy around my wife. How dare you all to betray me! I shall have you all demoted, except for Sir Lancelot, my dearest, truest friend. Thank you for keeping my trust, good noble sir. Now, what do you have to say for yourself?”
…But Sir Lancelot could not speak.