It's probably because Bonney is a child??? You huys stated it yourself. He does this to women. Bonney appears as an adult women but in this reading you said what Vegapunk stated about Bonney being a child. Sanji won't change and honestly I prefer it that way. While it tends to be an annoying trope it's still a part of Sanji's personality. It doesn't exactly make sense why his personality would suddenly change during the Egghead arc anyway. If he suddenly changed it most likely would have been during the two year time skip when they went back to Sabaody. But even with this falls in-love with every women he still makes up for it in other aspects.