Yeah, I could clearly see their relationship changing. But I don't think it will lead to anything good. I really do hope the story will move on, and change it's focus on something else since it seems to me that Team Gaji can't make Sumin x Sungji's love become stronger or deeper than it always is anyway. Also, we don't have that many chapters left for this endgame couple to really build such real trusts, and the thing is that it doesn't seem like they have ever needed one in the first place. And, I hope the author remember that this manhwa is not only about them; there are still some stories left to tell as well.
I don't think you need to be Seju's fan to dislike where the story has been heading. If you prefer interesting drama/plots, you would want to see the story develops around Seju more because Sumin x Sungji hasn't seemed to be able to bring anything interesting to the story other than just their sex scenes, I would say. And how is the way they made up considered as common though? Sungji caught Sumin sleeping with Seju. Wasn't she supposed to find it hard to go back to have physical intimacy with Sumin? And it's not the whole sex scene per se that I dislike about, but how they went all the way to do it without having any proper communication first, or any other better excuse to help convince that they weren't doing it just because it's been a while is. And it's Sungji's logic that seems so weird and inconsistent to me. Yeah, them acting way too much like horny teens is a thing. I don't follow discussions elsewhere, so I'm not sure about what people at other places have been talking about this chapter, but weren't most of them were like "This is the best chapter ever in the history of wdtfs.", "Sumin's just sooo sweet, and Sungji's sooo cool", "I have to change my bed sheet because of this chapter.", etc, etc. They didn't seem to talk much about how they like about where the story was heading though.