You see people didn't bathe much. Including the clergy. Hence everyone had trouble with body lice. Now if you were married, your betroved would shave your balls as protection angainst critters that could possibly carrying the plague. Priest don't get married and who blames em for not wanting to trust the 14 year old altar boy they're "tutoring" with a razor, right? Therefore the nuns had to perform this service to their shrivled old gray pubes.
As a history buff, I find the comparison right on topic.