Sanji describes his time on Momorio Island as "hell". Despite what he lets his crewmates believe it has nothing to do with the lack of ladies. He spent two years either fleeing or fighting. He did not come away unscarred and did not have a fun adventure. An attempt at a canon compliant explanation for Sanji's reactions without just being a prejudiced jerk. Lots of trauma and internal narrative.

“Two boys can have sex, too?” Luffy grins as he says it, like this is a normal conversation. “No,” Sanji hisses vehemently at the same time that Zoro says “Yup,” grinning at Sanji with way too much teeth and seriously--who smiles like that? Luffy laughs and slaps a hand down on one of Zoro’s shoulders, leaning in. “Hey! Let’s have sex, then!”