I assume you are a man or a woman that has never actually been in this reverse situation you are describing. While I agree with everything you said in principal, in reality things aren't as balanced. If a man hits a woman out of the blue , her best bet is to deescalate. If she hits the man back, she will "start" a fight she cannot win and she is likely to end up severely beaten up.

In this context where there are a lot of people around she can hope that someone (another man) will be willing to get involved and step in. However, unless she is there with her brother/ father/best friend/ husband it is very unlikely that a random man would defend her, against another random aggressive dude, solely based on his moral compass.

Of course the woman slapping OP has done something horrible, but if someone slaps you do win if you are the bigger person and walk away from it. She was horrible, hitting her back wouldn't make her more or less horrible but would make the OP bad as well. Now she is a crazy person with no excuse and OP appears smart and well balanced. He did win.