People, lets think for a moment. Tashigi is complaining a lot about sexism. She assumes Zoro is discriminating her basing on her gender. People assume things basing on their previous experience. It is likely, that some people displayed analogous behavior towards Tashigi, as Zoro is now, justifying their actions with sexist convictions. If she had to put up with this often, it's no wonder she is overly sensitive on the matter. So far we have only seen her dealing with "positive" discrimination, which was (for her) aggravating enough. However it's almost inevitable, that she had to put up with worse kind too. Just look at the Marine Corps. If I'm not mistaken by this point there were precisely five female Marines (apart from Tashigi, Tsuru, Hina, unnamed Giantess and formerly Bell-Mere). There is even more pirate women, than marine ones (apart from Nami and Robin, there Alvida, Perona, Lola, Bonney, Big Mom, Boa Hancock with her all female crew, Whitebeard had three female captain allies and there were some women in Bellamy's crew). If the line of work is so much dominated by men it's certain sexism would occur, moreover every female has to deal with it on her own, as there are so few of them, they hardly can meet on regular basis and maintain contact and support - meaning no safe environment to blow off steam (resulting from aggravation cause by sexist antics). Furthermore, as she trains Tashigi probably has simply different pace of development, than other marine swordsmen, caused by the fact that she has different physiology, so on some occasions she may seem much weaker than them, resulting in them looking down on her. On the other times, she may seem much stronger, and develop quicker than them, but since she is sort of an outsider in predominantly male society, she is given less acceptance than others, which may result in her peers being jealous of her, rather than in awe of her skills, maybe to the point of accusing her of using some cheep tricks. It's a sort of thing Kuina has to deal with. In fact in her life she probably was more likely to be praised based solely on her gender (see G-5 marines) - which is not so important to her as it's a factor out o her control, than on her skills as a swordswoman or capable officer - and it's natural for one to feel bad if they put a lot of work into something and are quite good at it too, and nobody is appreciating it. Might be also one of reasons she sticks to Smoker - he treats her as if she had no gender.
It's just a hypothesis, we won't know it it's true, since we are presented with flashback dealing with that theme.
I don't think Tashigi is right accusing Zoro of sexism, but I can rather well understand where it might be coming from and honestly I can't blame her.
When Zoro states, he is superior to Tashigi, he only means, that he is superior as a swordsman, because of his superior strength and skills, not because of difference in gender. Zoro, probably can't thoroughly picture the situation Tashigi is in, and he likely don't even care much.
Sanji is different case. He is sexist, but sexist in reverse. He seem to think females are superior beings by default, regardless of their combat strength or even physical beauty (although he only personally appreciates beautiful ones). And lesser beings men are have simply no right to rise a hand against them in similar way common people can't rise a hand against noble man in many feudal societies. It's still bad, as women are obviously no more superior to men, than man are to woman, and there is no virtue in simply belonging to one sex or another.