That's because you don't choose to be offended by it. Taking umbrage at a simple word is a choice that feminists (in this case) consciously make. Would they be offended at being called a prick? How about a "knee" or "wrist"? Those are body parts. Being called a piece of the anatomy is only objectionable because the target decides to take exception.
Honestly, I could see a woman being offended by being called "ovary" or "Breasts", because the speaker is reducing the woman to her biological role in reproduction, which seems calculated to objectify her, but the word ****is so divorced from the actual anatomical meaning that it's hardly even an insult, much like "prick". Except by consensus, **** is no more offensive than "putz".