A coward is someone who fails to accomplish or fulfill a certain thing through the idea of fear for a certain situation.
In this situation, if I hit a girl, I am not really a coward. I am more of someone who has to resort to hitting someone to prove a point, but this is not the same when it comes to defending myself.
If it's on some small degree of something playful like a little shove or tap on the chest then that's completely fine. I get used to that all the time anyway so it's no big deal and that mostly just indicates something friendly.
If it's at the level where she's trying to kill me and attack me then I will try to restrain her, and if she is still acting crazy then I have no choice but to hold my ground and either restrain her and hit her back. I see no other way for myself to handle someone who is trying to kill me if I don't either try to stop them or just attack them back.
--If it's for self defense in a serious situation then you are not a coward. You are just trying to protect yourself.
--If it's out of nowhere and you had no reasons for hitting her then you are still not a coward. You are just a punk.
The word coward can be used in different ways, like jumping one person with a group of people, or being afraid of doing something that needs to be done.
If I hit a girl (though I can't really imagine myself doing it) then it better be for a good reason, and not just off of some stupid reaction like getting drunk or hitting her because she decided to talk shit to my face. Those are not good reasons or reasons worth hitting a girl.
I would only come to the conclusion of hitting a girl if she has an object that could kill me. Unless the girl is much bigger than me and she starts swinging at me, I'm not going to hit her.
But I do agree with that experiment, that if a girl hits a guy in public then it is ignored and people just laugh, but when a guy hits a girl then it is taken seriously and we are regarded as scum when the situation reversed is not the case.