Because to anyone outside of your male-centric perspective, using how girls throw to imply that someone is weak or lesser, is insulting to anyone who isn't male. It is antiquated, derogatory, and frankly, offensive as f***. I would wager that any girl in high school or college softball can throw better than the majority of non athletes, so the analogy in your insult even lacks merit to begin with.
You also claim to detest conformity, but here you are being upset that a message board won't conform to your antiquated notions of what is acceptable speech, over a sexist insult that is no longer in common usage. Do you not recognize the disconnect here?