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Sanji is born imperfect.
Judge deemed him a failure for being born a girl, he didn't need two girls when all the natural strength of a man would've been preferable. Not even the modifications took, so he was a failure on two ends. That, of course, did not dissuade his brothers from bullying him. Staying your hand was what weak men did, his father wouldn't stop them even if he begged and cried. Snakes can shed their skins, but in the end they're still just a snake.
Still, Sanji tried to be the son his father wanted, to be strong despite how he had been born.
Sanji's mother didn't care, she loved him all the same. Loved them all until she was dead and buried, not a single rose left by his father, bouquets gathered by himself. Even when he stood out in the rain and plucked petals from it, red running down his fingers as he wished he had been born different. Stronger, a man, able to impress his father. Reiju showed him mercy when she helped a young Sanji escape, further driving it into his head that women were the merciful. The ones who would throw him scraps despite all his imperfections, the mantle he had to take to fit.
So how could a failure like him be a woman? Strong and merciful. Women are perfect, soft, standing above him in their ethereal beauty. Not built to toil and fight for scraps.
Sanji was so obviously a man, what he was supposed to be, what his father wanted. He had to become stronger. So he practiced a deeper voice, called himself a man even after Zeff rescued him, and insisted on being called one. Zeff only huffed and complied, taking him to buy suits when puberty hit, shrugging after he began growing facial hair later one. Every injection cemented the man he was always meant to be. He'd never raise a finger against one in a fight, just like Zeff said. He had to be a proper man and forgive women, get stronger to protect them.
You are not a woman. You can't be one. They're too good for you: flawed and male. The only way he could be strong was to be a man, like he was supposed to be from birth. Everything he should've been felt like a suffocating blanket, as if trapped under layers of skin, layers of the men he could've been. Dead and buried next to his mother, to the men his brothers could've been, his sister...
Sanji had buried every version of himself by now, left flowers at the worn gravestones without looking back.
Ivankov had seen right through him, of course. Able to be strong in any form, something Sanji struggled to understand, went against everything he had been told growing up. The whole ordeal at Kamabakka had felt like hell at first, being forced to look yourself in a mirror even if you don't like what you see. The Okama used their identity as strength and were comfortable in their skin, their clothing and makeup - everything Sanji was not. He could wear a dress and still be a man, he wouldn't be weak even if he had decided to shed his identity and live as a women. But in the end, in the face of an uncertain future, Sanji decided to remain as he is. Even if Ivankov could've changed him, or he himself could've shed his masculinity.
Now Sanji was a man for himself, a cook and damn good - both everything his father would never see him as, everything he hated turned into a gift. Everything Judge wanted for him turned on its head, both his hates and wants mixed into the perfect slap in the face. He was a man for himself, he was strong for himself. Luffy had helped him realize that on Whole Cake Island, Sanji was their cook no matter where he came from or how he was born. He saw everything Judge did and embraced it, the whole crew was. Even Zoro did, valuing Sanji as his partner in every sense. A man and just as strong, not because of it, but because Sanji was himself.
Men can be kind. Zeff, Luffy - Zoro when he felt like it. Being a man wasn't a damnation, being a woman wasn't a weakness. Despite this, Sanji felt a comfort in his manhood, a natural fit, suiting him like a good tie. Or the way he fits perfectly at Zoro's side when they fight. No longer a flaw or condemnation of assumed weakness. It was a metamorphosis: a beautiful and deadly butterfly from his suffocating, limiting cocoon.
Sanji was reborn stronger and overcoming the shackles he had bore from his childhood. Masculinity isn't a shield now, it is armor not just to protect himself, and a weapon poised at all their foes. Femininity isn't a weakness, isn't wholly perfect, even if he still won't raise his hand against women.
He was born again and felt it was like truly living for the first time.