The past few chapters were a MESS! Seriously, fuck Sumin! I have been a fan of her since the start, even with that womanizer side of hers. I thought she was and wanted to become a better person for Sungji (based on how her personality started to soften every time she's around Sungji). I understand she still loved Seju but even though it was a goodbye sex, it was cheating no matter how you look at it!
Honestly, I don't really blame pink for doing this. After all, she's been crazy in love with Sumin for a decade, she's practically obsessed; so it's rather expected of her to be pulling this evil trick just to separate blonde and black.
About Sungji's reaction, well, Sumin was her first love after all. I don't think she actually knew how or what to react and so far she's appeared to be a calm and collected person, thus for her to lose control and go all psychotic is highly unlikely.