To some it seems that the lines Jitae said were corny or cringe but for me it was a genuine in the moment feeling.
Like it's an intimate moment for them. That they want to f*ck not just to satisfy their bodies but to confirm that they are on the same page now, somehow a physical proof that they will be together to surpass this. No more sacrifices, no more leaving someone out, just them.
For me the lines were not cringe coz If I were Mieum, I would feel that Jitae wants a confirmation that no matter the circumstances, he is there for him. that for so many years that he is careful with who he associates with coz maybe they're spies, he found a safe haven from Mieum were he can be vulnerable, were no one would judge him or stab him in the back. Just pure innocence, love, and comfort.
Idk, if the lines sound funny or unfit for you, as long as for me I felt what these two want to convey, I'm already settled as a reader. It's what authors somewhat wants us to see that regardless of the bad translation or lines, we ought to see the intent and feelings of those lines blurted out by the characters.