short spoiler:
Black Maria was inside the whore house, Sanji met her. Black Maria knew Sanji was an enemy but she does not want to fight him as she smells his okama haki.
Sanji asks her to release the whores so there would be no need for bloodshed, although he didnt really wanna fight women.
Black Maria and sanji argued some more until she got angry and attacked Sanji.
Sanji stated that he doesnt want to fight women, so Black Maria lifted up her kimono and showed her dick. She is actually an hermaprodite and used to train with Ivankov.
After seeing her dick Sanji no longer hesitated and kicked her.
Source: pimp who lives nextdoor