People here are saying it is justifiable in some way for a woman to slap a man because she is so much weaker and can’t do as much harm (MMA fighters excepted). Well, what if he does not slap her with full force? Say, he gives her a girly slap? I don’t think we’d consider that socially equivalent either.
Or, take the case of middle-school children where boys and girls are still about equivalent in strength. The consequences for a boy making the slap are so much worse than if a girl does the slap, both officially and socially.
Women slapping men does not empower them, it degrades them. It says about women: “ You can slap because you are safe to do so. You women are so weak, men can take it without harm. You are like a child”.
A man would not retaliate against a five your old who slaps him… or a woman. But, if I man slapped another man, it is a challenge and he should be expecting a receipt to the base of his chin. Men are equals. Women are in the category with children and the elderly. Not only physically, but they can also not control themselves, like the man who just got slapped is expected to. Not exactly empowering for women. This is the meaning of the trope.
Women should be more outraged by the female slap trope than men.