I didn't mean to get all ad-hominem. Some people literally can't differentiate between juvenile and non-juvenile humor. Apologies .

I haven't answered the question because the whole premise is flawed. You're asking why it's never appropriate to discuss some body parts, but that's not true.

Extremely personal real world example: I had a urethral stricture biopsy done years and years ago (I'm OK now). Discussions in that context get clinical and professional very quickly. Nobody is making any snickering jokes in the operating room.