yoonie
hehe.. but Sumin broke up with a girl while eating her out and immediately jumped to Seju's bed.. ch50.. also, she gave in for sex in ch55 WHILE dating Sungji.. so this part shouldn't be much of your worries XD (oh wait, didn't Sumin cut off the sex session with Sungji in ch81 to pick Seju up XD)
You cant make a comparison between Sungji and other exes and we know it. She is different. The sex in chap 55 is for her to cut off with Seju, that was Seju's condition, Seju shouldnt be the one who asked for it when she knew Sumin was dating someone else. Most of that part is Seju's fault about the gettting caught in chap 55 and Sumin always had soft spot for Seju, she would give in. If we were about to blame, Seju and Sumin are both jerks in that chap
Anyway, this is an ending, its not some kind of plot twist or building drama like arc 2's ending so I dont want it to be an ending where one of them falls into "jerk" category or half-ass ending. I want a justified ending. This is one of the reasons why I dont go for SejuxSumin ending with the current events of wdtfs. Making Sumin come back to Seju right after Sungji left, even by Sungji's own will of realizing that Sumin might not be in love with her but with Seju or sth, it feels off. All the guilt, all the love, or maybe even pain when Sungji goes away suddenly disappears and she jumps right back to Seju.
I'll give this ending more thoughts when more events are revealed but as of now, honestly, SejuxSumin's ending is still a far question from another world XDDDDDD. Sungji might or might not leave but that doesnt make the other two a couple right after. That's a different story.
To make SejuxSumin an ending, its not impossible but hard as heck with this pace of the story and Sungji.
Sungji has all the needed pieces of the puzzle
Sumin, not yet
Nammi's appearance is to ensure the following:
1- she knows about the current arrangement and situation
2- she knows about everybody's feelings
3- she is no longer interested in Seju (getting public and serious about Bang Uri) and Seju is clearly annoyed by Nammi's presence
4- making sure that readers who didn't realise that the doctor in ch38 is in fact Nammi's father are now aware of this info clearly! (he is a surgeon and probably the one who was there when Sumin first got her scars)
connecting the dots is as usual, Nammi's job.. ch16, ch19, ch49, ch65 (and ch50 at least in Sumin's mind)
Like I said above, Sungji leaving, Sumin realizing her love or sth doesnt guarantee an ending for SejuxSumin. Its not. Seju and Sumin are in a state where they might both know they're in love with each other but to be back to be a couple again is not from that same equation.
This is what I draw out from the current events, I'll give this couple ending more credit if things changed in the future.