I think Sumin and Seju still love each other, they are denying it. In my opinion, it's impossible for you to have sex with someone for a long time without any feeling, beside they are ex-gfs. Sumin just still love and hate Seju at the same time. That's the reason why she is still hesitating to do something sexual with Sungji, she feels like shes cheating on Sungji with Seju. May be she really likes Sungji, but the one she loves is Seju.
