Because you are using an intimate part of a femail body as an insult. Really no different to calling someone a prick, but is very sensitive to some women.
Ah you see, you fell neatly into the trap I laid for goonercymraeg, where my comeback was going to be, I think you can tell I was taking the piss even without the
!I would celebrate my victory but what with you being a Spurs fan and all...