Historian Kate Simon has a rather naughty story about that in her
A Renaissance Tapestry: the Gonzaga of Mantua. It would appear that a certain noble feller had to prove that his marriage wasn't annulled because of his impotency, and so had to demonstrate his, ah, ability, with a paid volunteer in front of a board of witnesses... Yes, as one might imagine, there was a certain amount of performance anxiety, not to mention that the volunteer demanded a retrial. And one of the requirements was...how shall I put this politely? - manual verification of the act in progress.
We think we got it bad... ;-)