My god, I don't get the whole "Seju can't get over Sumin cos she keeps coming back for more" argument. It's time for one of those classic phrases...."It takes two to tango"! If Seju really wanted to move on then she could have refused Sumin.
To me, Seju seems so self-centred. That scene where she says "It's already been a month" in reference to Sumin's parents death seemed to me more like "It's already been a month, now forget about it and turn your attention back to me". If anyone has been unfortunate to lose someone close to them then you will know that a month is not enough time to mourn.
Also, the "she only cheated once, she deserves another chance" argument is ridiculous. It devalues the whole point of a relationship. And Seju wasn't completely hammered when she slept with that guy...Sumin called her out by mentioning the security guard. Sure, Sumin is no angel but Seju pushed her to be that way.
Sigh rant over.