Zoro’s backstory involved a girl who wanted to be the strongest swords(wo)man, who hated how no one would treat her as an equal or even consider that it was possible for her.
They would always put her on a pedestal, “women can’t be fighters”, etc. It drove her crazy when she thought she couldn’t achieve her dream just because she was a girl.
So every time Sanji treats women as someone to be protected just because of their sex, that reminds Zoro of how the exact same attitude hurts his childhood friend. But every time Zoro treats a woman like an equal who is just as capable of fighting as any man, Sanji gets upset that he would dare consider striking a woman.
Even their very first fight stems from treating a woman (Nami) as either someone to be protected or a credible and capable threat.
And that fight set the tone for their whole relationship, which was only reinforced every single time one of them interacted with a woman in front of the other.
Every single time Sanji puts women on a pedestal, it (to some degree) insults Zoro’s memory of his friend, who hated that very attitude until her dying breath. No wonder they don’t get along.
*Theory by BlueberryPhi
