To be fair to the FMC in that regard, and I could be wrong since I still haven't MTL it
The relationship between the second couple is all but dead, I wouldn't be surprised if they broke up after the trip. The second girl has no interest for her BF, and the same thing for the BF to the GF. Also, I do like the contrast between the MC and the Second Girl being more romantic and emotional whereas the FMC and the Second Dude is more of the same with a debauch kiss.
The problem with the MC is that he wants to please everyone, which is why his relationship with the second girl seems more romantic. She looks for in him what her boyfriend hasn't given her, and they do things that she would like to do. I don't see it as romantic because he is also thinking about what FMC might be doing.
I don't think that FMC and the second guy are more of the same, he is violent and antisocial. They have a date and when she tells him she's not going to the hotel that day, he says “goodbye” and leaves.It's also a contrast to what you say about the relationships between these exchange couples. While the MC tries to please the second guy, if he doesn't get what he wants, he gets angry. I'm not saying FMC is a good person either
In my opinion, the MC should decide what he wants in his life, but I think that in the end, the MC and FMC will end up together somehow, the MC with another girl and the FMC as his lover, a kind of double life that would fit the FMC's fetish.