This rule was employed on H-B's Pac Man cartoon. Whenever Pepper, aka Ms. Pac Man was involved with the adventure, it would always be her, rather than Packy himself, who chomped Sue. Packy would only chomp Sue when she was paired or teamed up with 1 or more of the male ghosts.
It goes back to what Silverstar said earlier; female characters are generally considered to be smaller and more delicate than males and so males cannot strike them for comic effect, they way that a female can strike a male for some petty, trivial reason. If a man ever hit a woman in a fit of petty anger, you wouldn't laugh at that. You'd think the guy was a hot headed creep. On One Piece, Nami can beat up on Luffy and the other Straw Hat pirates for comic effect and it be funny, but if Luffy ever beat up on Nami or Robin in the same manner, you would hate him instantly. The male Straw Hat pirates like Zoro and Sanji are strong enough to lift houses, so they can take it if a woman punches them on the arm or smacks them on the head.