BOTH of them are emotionally constipated and don't know what to do with their feelings. That's why the last few chapters before the sides were so good because these two actually communicated for the first time and so openly at that. I understand where you're coming from with the hate towards sex but to me that slow burn is the exact reason why I love this Manhwa so so much.
To me that's the point, Minchan likes sex to feel warmth and to feel good just for a moment in his life, it's what he does when he feels shitty. Their sex scenes show just how much their relationship progresses, I wouldn't say they did it through every inconvenience but for very impactful moments they go through. They have gotten more intimate and caring for each other through that bonding, and Minchan is noticeably becoming more open with Bihwan which will lead to that eventual trust and acceptance.

Reading the sex scenes in this manhwa make me feel icky. It’s like, instead of being mad or being upset or sad and sitting in those feelings, they have sex. It’s like, just be angry, be upset, you don’t need to fuck every time you have a bad feeling.