Here you go:
Not all jokes or conversations about one particular body part are juvenile. The one in question was. I don't think any reasonable person would dispute that, even the guy who unfortunately lost his job over it. He's not exactly standing behind his joke as the pinnacle of human creativity and expressiveness.
If your senses of decorum and humor have not yet matured to the point where you can tell juvenile and inappropriate behavior from the sort of thing that is OK in a professional context like an office or a convention hall, I legitimately feel sorry for you. It must be very difficult to go through life like that.
You still haven't answered the question though. And do please refrain from the ad-hominem arguments.
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.
My mistake for being ambiguous.
I don't think I'm misunderstanding you at all. You said that "juvenile" humor is pervasive. Wouldn't that indicate that the general consensus is that this type of humor is OK?
Let's get it straight - the (very small) minority here want to make the environment more comfortable for themselves.
(Also, nobody has ever explained to me why a joke or even a conversation about this one particular body part qualifies as "juvenile".)