Zoro is a cambion, which means he feeds on sexual energy. A small crew like the Strawhats, its not normally enough to keep a cambion satisfied, but Sanji happens to be an abnormally sexually driven cook so it usually works out okay for Zoro. At least, it works out alright until Sanji finds out that Zoro's been taking energy from him through his dreams. For Sanji, he's trying to wade through the confusing mess of emotions for a certain marimo after a surprise kiss on the battle feild. It's not easy to do on a normal basis. It certainly isn't any easier when the man he has deeper feelings for is a cambion. Then everyone learns how important it is to keep their local cambion fed, for if they don't, someone might get eaten. literally.

A lifetime of hunting demons never prepared Jon for suddenly finding himself the fake ex-boyfriend of a man with incredibly conniving adult children. Jaster, for his part, got in over his head about the time he lied to Arla and Jango about where his hickey came from. The demon hunters are just a little extra depth, at this point.