I don't know.. It's up to the author. And how sungji will react about this infidelity is the turning point between all the characters. It will decide the ending. for me, In ch 50, "if things can't go back, u have no choice to play the fool"
In a way, I feel sumin still loves seju more than sungji. She scared about the possibility of seju suicide, she still cares about seju, even she doesn't want to admit it before. But she can't forgive what seju and her family did to her (and maybe her parents) , so going back together isn't an option for her.
Sumin and Seju have had a long relationship. Minimal 17 years... and I am just counting when they are couple not before (We know they met when they were children). For me that is the main factor because they have not continued with her lifes apart. Seju has loaded the guilt of being unfaithful in a moment of vulnerability for Sumin, also, we cannot forget what her uncle did. Since that moment, she promised to stay with Sumin until the death.On the other hand, Sumin decided to live a empty life, without anybody whom she could love as she did it with Seju.Sumin created this as a defense mechanism. She prefered not to have feelings in her relations, but at the end, Sumin is just human, and when she needed to feel something, she came back with Seju.
I think Sumin was confused because for the first time since Seju, Sungji is somebody important in her life too. So she needed to take a decision, and this was Sungji.Maybe Sumin was tired of a toxic relation with Seju, so she saw Sungji like her solution, a new life, a new lover, at the same time than she wanted than Seju could understand their relationship was not good.
In ch 55. I think sungji is the one who said "think has changed". Sungji has been insecure whether sumin loves sungji or not. But that thought has changed, now she has more confidence about the relationship.
She said it can't end the way you wanted to, to seju. She declare a war. How sungmin ask sungji not to leave in the bus stop... Means something.
Sungji is not going to back out, but I think, Sumin is who has to move. I just hope Sungji does not give all the solution. Sumin cannot be a toy all the story!!