Seo & Youngchan have been using sex with each other as an unhealthy coping mechanism this whole time. They both have control issues, self-esteem issues & are codependent on how the other makes them feel. They're just bad for business.
Depending on how the story ends, this very fact might make their end goal and resolution so much more fulfilling. I hope both can really open their eyes to the Situation and learn to grow - cause they are complimentary to eachother in the long run.
...I might actually cry the first time they sleep together. It's going to be 100% lovemaking & I'm here for it.