My opinion
Isn't this whole story based off of Sumin after she get's cheated on by Seju for being depressed after the death of her parents? I don't think Sumin would learn her lesson if Sungji cheats on her. In fact, it'd probably turn Sungji into Sumin 2.0. She would then become as 'void of emotions'/love averse as Sumin, hook up with Sumin and other women and be haunted by the sight of Sumin and Seju together. She'd hold this over Sumin's head. Where's the healing in that scenario? The only way this healing thing works best is if Sungji dumps Sumin, accepts what had happenned, moves on and dates Dawoon her possible high school crush(?)
All in all, I'm glad they finally decided to give us more about Sungji's past through Dawoon. I knew she wasn't a one dimensional character from maybe chapter 7 on.
No lie, she did come off like a total Betty Sue (perfect character) at the beginning but it kind of changed for me especially when they mentioned in one of the earlier chapters that there was a possible rumor about Sungji being into women or other 'weird shit' from back in high school see chapter 7 .
Also, when Sungji cried(?)/ got really anxious when Sumin was making moves on her in the early on without them being in a full blown relationship (she wanted to make sure that they both loved each other to some degree).
Heck even when they first slept together, Sungji was kinda panicking and wanting to see Sumin's face during sex just to feel safe showed that there was a lot more to her than her looks and her normally boring dialogue.
From the looks of it, her relationship of being pretty direct with Sumin (like telling Sumin she likes her, etc) came from learning her lesson with being indecisive with Dawoon when she was confessed to...)
Anyway, just wanted to express my opinion and I can't wait for the next chapter