I swear, we seem to have these threads every month.
Nathaniel Grey said:
from my grandmother who gave me explicit directions that if anyone ever messed with me I was allowed to knock them out.
First, who would teach a child this? This seems like poor guidance to me. "If someone calls you names, Timmy, feel free to unleash your fists o' fury". In fact my own father taught me almost the opposite. So I have a feeling we're going to reach an impasse, OP.
Nathaniel Grey said:
I'm looking for a logical argument.
I suppose mine would be something like this:
Women have roughly half the upper body strength of men, on average.
Women are roughly half a foot shorter than men, on average.
Women are roughly ten to fifteen kilograms lighter than men, on average.
Therefore, women are less likely to pose a physical threat than men.
In cases of self defence, it is generally required for the defender to believe that they were in danger for the defence to be justified. This is less likely, although not certainly, to be the case, because of the previous arguments (and almost definitely not the case if the woman is armed).
In cases of non-self defence, moral outrage is generally increased if the victim is unable to defend themselves or weaker than the aggressor, for example with kids etc. Furthermore, damage is likely to be higher. You can conceivably kill someone smaller than you with a single punch to the head.
So clearly, "don't hit women" doesn't apply in all circumstances. But, again on average, hitting women is going to be less justified/more dangerous/more morally outrageous than hitting a man.
It's the same sort of logic behind weight classes in boxing. Even highly trained professionals are liable to get their asses handed to them if there's so much as a 5-10 kg difference between them.