I found quite weak the main couple, like the only conection they have is sexual, and superficially emotional ( i don't see the reasons why sungji loves sumin, it seems special because she never develop feelings for anyone else before even she had a lot of suitors)
Sungji is so devout to Sumin for the same reason Sumin used to be with Seju: First love feeling. The thing is Sungji and Sumin had the same kind of feeling with 10 years apart.. But SejuxSumin problems, love, sexual interactions, cheats are more intense and personal they talk about they feelings ( good or bad).
Sumin tryng to move on and never worked out it's not because of Seju's actions is because Sumin is the weakest part of the story, she always look after Seju no matter what. She took revange over Suju betrayal, she flirted, had relationship with girls she later dumped. then she came back to Seju saying " i don't know why".
Sungji reaction and forgiveness over Sumin cheating on her was so easy, even a little lame, the thing is Suju cheated Sumin with a man, yes, a stranger, Sumin cheated Sungji with her ex girlfriend ( and boss).
So i think the story is more about two point of view about love on different stages. SuminxSungji is Fresh, new and happy with no problems at all ( no even with a cheating). Sejux sumin is a tired love intense but quite toxic, like in any long term relationship.