Jejeje, I saw it on the raws first, and I thought the same as you without context, but they're actually doing something similar to dual cultivation in order to heal and share powers. It also has something to do with that magic water that Jae carries with him. When Ryeo drinks that water is like an aphrodisiac, and he also needs Jae's cold energy to be able to handle the power of that water, something like that (￣∇￣")
Yeppp
The sex isn't solely for pleasure, but for recuperation. They use the water from winter palace to recuperate their body, even fixing their tattered soul. But the water has side effect; the urge of needs and heat that need to be released. Sex is the option for that release.
i love how its a fuck or die situation
Very convenient (≧∀≦)
I love them but how is sex the first course of action after basically dying and still being mortally wounded