It's a custom that involves magical protective underpants.
That gives it a high comedic value.
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The fact is that the underwear is just an item like a crucifix that you wear as jewelry. They remind you of who you are and what you have promised to be. Can you imagine committing adultery if you had to strip off a large crucifix before you committed the deed?
There is no protective value to them that I have ever heard of espoused by church doctrine.
The reality is that the underwear is really the same as everybody elses was in the past it is just that the styles have changed and the “underwear” they wear hasen’t
It isn't that comedic..