to them, its a fubu relationship. jaeyi knows that he isnt like this with the others (his friend even said he treats his exes worse than ideum) and that's saying smth.
ideum doesnt have an inkling of what or how a fubu rs is supposed to work, so he just goes w it, not knowing that whatever jaeyi's doing (his affections, consideration, and whatnot) is more than what a fuck buddy should do
that actually does open a new perspective now that you put it that way!!
i guess you could also interpret that because jaeyi has never felt this way (as you said, has noticed he never acts this way towards anyone else, even when he’s in a relationship), i don’t think he realizes that that’s how people act when they are starting to like someone. i think it’s easier for him to think, wow! we have such great sexual chemistry and i am feeling this way because of that.
on the other hand, ideum also doesn’t realize that they’re blurring the lines between fubu like you said, and just wants to go along with things to keep things going, as you said.

i feel the angst coming
jaeyi it’s not even about the sex anymore you like the man just realize it before the regretful tag top comes into play