-Most likely a gag since it's irrelevant in the grand scheme of things. Zoro also literally asked for it, he is fine. Sanji is lowkey helping Zoro to get stronger. Another proof of their rivalry?
-Reasonable, given he made a decision aiming for a greater good based on all the info he had at that moment. Even Luffy admitted that the pain Sanji was feeling was way higher than getting hit like that.
-Robin is a pirate in the Strawhat crew, she is already way too exposed. Sanji showed great sign of trust, even in a much weaker crewmate, and he recognized his mistake by getting caught (rare behaviour to most characters), even saying sorry. Robin thanked Sanji to death cuz someone finally accepted her as a strong and competent combatant, and not some deadweight who needs to be protected by everyone even when it's not possible.
Yeah, Sanji could've defeated Black Maria with ease so Robin wouldn't need to fight her (but she also wouldn't develop so much as she did), BM herself admitted it, but that goes against Sanji's code and values (that also comes from his most utter admiration for Zeff). Which then became another dilemma for Sanji: What version of him would be more useful for the King of the Pirates?
Obviously Sanji choose to remain as he is, he isn't changing completely who he is because it might or might not be the most useful to Luffy anyways, and he most likely thinks that Luffy would prefer him to remain as the Sanji we know. He even asked Zoro to kill him somehow, if things went out the worst for Sanji himself..
Which shows us two things:
-Sanji has absolute pride on who he is and in his values, he doesn't want to live a life where he can't be who he wants to be (Conqueror trait in fact)
-Strawhats already got indirectly benefited by Sanji's so called weakness. His kindness and the fact he doesn't hit women made Sanji and the crew indispensable allies (who were enemies previously), such as Violet and Pudding (only counting the female ones)
By the way, im not trying to slander Zoro here, just showing some people's hypocrisy

