That's a part of his character and basic principles, lol. He can't hit a woman, it's as simple as that. Call it selfish or whatever you can. He's willing to die for his Nakama like he did while saving Usopp and Nami from Enel but he can't hit women. You cannot call him 'weak-willed' for that. Again, the logic used is bad. I would even say that him breaking his principles and beating a woman is a sign of him being weak-willed, towards his principles or in general.