"What if we're talking about kids, is it OK to punch back a kid?"

Imma stop you right there. Kids are neither physically nor mentally developed, and cannot be held responsible for their actions all the time. Women, by definition, are adults and should know that there are consequences to their actions. Do not try to pick a fight with a person tougher than you. That is, if you're too cool to not pick a fight at all, like a decent human being.

And yes, a lot of the time people do mind retaliation if it's man hitting a woman back. The whole crowd around jumps as soon as a man hits a woman, with no context. I've seen it happening myself. The man was getting kicked by the woman's heels, onlookers looked at it entertained by the supposed "humiliation" the man was facing. The man grabs her leg once and pushes her back, and tens of people jump on him and beat him up.

I would have understood if you said something along the lines of "do not hurt weaker people, you'll probably hurt them worse than they can do to you". But your points are just wierd.

And I've undergone this myself. I fought back against a girl 2 years older than me, when she tried to bully me and my friends. She cried and told the teachers that I had grabbed her breasts to start the fight (which I didn't, even pervs wouldn't want to do that to her). I would've gotten a permanent record and expulsion, had my parents not come and cried for forgiveness as I stood there in the corner unable to grasp the gravity of the situation.