The following isn't intended to be against or pro any of the teams or any character, but rather, an intuitive reading of the work and understanding the significance of the "getting caught scene", as you know, this scene was planned long before the serialisation and never really changed because of its importance. Here is one way to look at it:
The concept of sex as a manner of testing connectivity:
Team Gaji intreduced this concept in earlier chapters and used it later in another more crucial one:
ch48, you see Seju telling Nammi: "if this is what you want to try out, do it, I will play along [..]", also she said: "don't kid yourself, this doesn't change anything". Nammi did want to test this thing by saying: "aren't you in fact scared, that you might like it?" but she struck out.
- No emotional connectivity or whatsoever.
ch49, this concept is still in act, take a look at the flashback of Nammi and Sumin. "it was far worse than I thought”. The outcome was also discussed in ch65.
- No connectivity once more.
ch54, Seju:"Just once, let me do what I want.. just once, it doesn't mean anything anyways"
pay attention to her words, it hints that Sumin is always in control to a point that Seju wasn't allowed to touch Sumin the way she wanted to in the first place.. take a look at ch14: "you are pretty complaint today".
Now, do you think the sex scene between Seju and Sumin in ch54 was just to mess things up ?
First: in the past 10 years, it's the first time they do it while Sumin admitting her feelings, it's no longer animalistic or suppressed emotional wise.
Second: does it really mean nothing?
look at the position, Seju is on top. There are three messages you could get from that scene alone.
- from Seju to Sungji "she is mine"
- from Seju to Sumin "it DOES mean everything in every way"
- from Seju to herself: " reclaiming.. feeling intimate one more time in 10 years".. and of course "now I know" ch57.
as for the first point: Seju looked directly into Sungji's eyes while biting Sumin's lips.
as for the second point: you could see the facial expression and no need to explain it further.
as for the third point: ch57, "I'm going to get you back"
Bottom Line
using intimacy to test feelings of one another was used in earlier chapters than in ch54. And the sex in that chapter wasn't a mere horniness, or a way to sabotage Sumin*Sungji, but also needed to prove something else to the three characters.