WARNING: The following contains spoilers for One Piece, Chapter #1,005 by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English now from Viz Media.
The women of One Piece are not your typical damsels in distress. While they have their weaknesses and problems, they're not to be taken lightly. However, this lesson seems to elude Sanji, as proven in his most recent encounter with Black Maria. His love of women, while well-meaning, is his biggest weakness and oftentimes gets him into trouble rather than helping get women out of it. This part of his character has fans of the series rather irked at him.
This isn't to say that Sanji is an awful character when it comes to his treatment of women. Rather, he treats them more like glass figurines than actual people. He has it in his head that being the hero to some beautiful woman may lead to romance. This is rarely the case and his old-fashioned ideas about women are used as a tool against him. His refusal to fight women has put his crew in danger, especially in the most recent chapters of the manga when he called out for Robin to rescue him from Black Maria.
Rather than fighting Maria himself, he couldn't overcome his outdated ideas about women and it put not only himself in immediate danger, but Franky and Robin as well.
Sanji's ideas about women aren't the worst. He believes they should be protected if they aren't strong enough to protect themselves. Not everyone in One Piece is a skilled fighter like the Straw Hats, so sometimes the occasion calls for such ideals. However, he refuses to fight a woman, even if she's trying to kill him. This is most likely due to a threat from Zeff the Chef who said that he would remove a certain body part and kill Sanji if he ever hit a woman. In that same sentiment, he referenced himself as a father to Sanji. Given the nature of their relationship, this had a huge impact on Sanji, and he took those words to heart. Many of Sanji's ideas about women come from him.
On top of this, Sanji constantly turns into a lecherous mess when in the presence of attractive women. There's nothing chivalrous about ogling and objectifying women, especially those who are a part of your crew. While some women like this kind of attention, Nami and Robin have shown they don't care for it. Part of being a gentleman is knowing when to respect a woman's boundaries, which Sanji constantly disregards.
While he may not physically harm a woman, his goofy antics do more harm than he realizes, especially to his crewmates. In Chapter #1,005 he was captured by Black Maria when one of her underlings feigned being sexually harassed, knowing about Sanji's penchant for trying to help women in trouble. This was all so he would call to Robin for help in order for Black Maria to capture her to read the Poneglyphs.
One Piece creator Eiichiro Oda has talked about Sanji's "policy" for not attacking women. He said in an interview that it's not that he won't hit a woman, but rather that he can't hit a woman. Sanji's pride and upbringing with Zeff, therefore, keeps him from raising a hand (foot?) against a woman. In his fight with Kalifa, Oda emphasized that Sanji was frustrated at his inability to fight a woman, and Nami recognized his feelings and praised him rather than tearing him down even further.
This disregard for his friends' safety has become such a problem that fans are wondering whether Sanji will ever overcome this mentality. Some One Piece fans don't think Sanji will ever hit a woman because it's part of his character, referencing the interview mentioned above. Others, meanwhile, think that if Sanji is to develop more as a character, he needs to overcome this idea. It's holding him back to the point that others are suffering.
Sanji isn't a badly written character. He's one who's struggling to overcome ideas that were drilled into his head to the point that it keeps him from doing what he needs to do to protect his friends. While his treatment of women isn't what one would call respectful, he has standards that he tries to uphold and he helps women in trouble, whether they are young or old. His heart is in the right place, but the toxic masculinity that Zeff instilled into him has done more harm than good. In a relationship, you should never raise a hand to your partner. But the women Sanji encounters on his adventures with the Straw Hats aren't romantic partners. They are women trying to actively harm him and his friends.
In order for Sanji to grow as a character, he needs to recognize that it doesn't make him less of a man for kicking a woman -- who's trying to kill him -- in the face. Hopefully, the fight between the Straw Hats and Black Maria will help him overcome this mental block and help Sanji realize you can be a gentleman and still roundhouse kick a spider woman.