I personally don’t really care for the sanzo dynamic where Zoro assumes he’s topping only for Sanji to unexpectedly top him. I prefer it when Sanji assumes Zoro would want to top and Zoro’s like “I really don’t care either way.”
I think it fits with Sanji seeing Zoro as this ideal specimen of masculinity everything Sanji feels like he isn’t, everything that made him a failure and therefore assuming that Zoro wouldn’t want to debase himself by bottoming. However, while Zoro fits the socially acceptable idea of what a man should be, it’s purely coincidental. He’s unapologetically himself regardless of what anyone around him thinks, and he doesn’t measure things by how they might appear to a hypothetical outside source the way Sanji does.