Then use your words and say "dude, stop" or "could you maybe turn it down?" If the DM let it go on and never did anything to stop it, then it's the DM's fault it got as far as it did. Just because someone else is a villain in the story doesn't mean you're not.

And this is in the hypothetical situation that the bard is the specific strange kind of person who learns of a possible gloryhole in a TTRPG and uses it without question.

All I see is a DM making a castration joke, which is a dick joke but more gruesome, while blaming a player for a situation entirely within the DM's power to stop by any number of peaceful, less disruptive means. They could have spoken to them, but they chose to cut off their dick.