I completely agree with you. That bothered me sm. And I hate it how they never try to talk it out. It happened twice and he felt hurt and left out twice. And not only that, but the fact that they lie to him makes it seem more wrong despite them being in a poly relationship. Almost as if it's "cheating" when it's not, cuz they all have the acknowledgement that they're in a poly relationship and consent to it, I really wish that if they were gonna do that twice, jihan should've also had his alone moments with jooin. Tho Even when he did at the hospital, they made it seem so unromantic and it only had like 3 panels and then it was done, while ikyoons lasted. Atp I think the author definitely had a favorite. Not jooin but the but the author .
I liked this overall but I don't like how the situation in the side story was handled at all. Jinhan gets jealous and expresses how he feels uncomfortable with jooin and ikyoon having sex without him, which they have done twice now, and for that ikyoon gets offended and starts fighting him for no reason, and then jooin puts them both in time out for "fighting" and continues to punish them until they "make up" but Jinhan's concerns are never actually addressed or acknowledged.
He's just "punished" for bringing it up and for ikyoon getting pissy about it. If this was real life, that's a serious communication issue in a 3p and could lead to a lot of issues down the road, including resentment by jinhan at such treatment, and more of jooin and ikyoon disregarding Jinhan's feelings because they can just steamroll over whatever happens.
Idk I'm probably just being dramatic but since this exact thing has happened to me before (in a friend group) I'm probably just being sensitive.