That was one of the trigger warnings I actually forgot to add for a solid 20 min and panicked when I saw it wasn’t there 😭. As for your question, in the chapter it mentioned that Chan was cheating behind Jisung’s back and telling Seungmin they were in an “open relationship” and then Jisung chose to stay with him. So morally Seungmin didn’t know any better. As for Jisung staying with him as much as it sucks to say and as much as neither of us might not do it in his stead there a lot of people choose to stay with their partners after they cheat. In this case Jisung stays because of literal years of emotional abuse and manipulation. Chan diminished his self-worth to being so non existent that Jisung felt Chan cheating was the best he was ever gonna get.

The justification for Seungmin is that he didn’t know and still doesn’t know that Chan was cheating, he thought Jisung consented because Jisung played along with the open relationship to keep Chan.

It’s important to remember that Jisung date Chan for 8 years. In those 8 years Chan abused him so much he won’t react like a regular person. Even more so saying that “he should’ve just left” is what a lot of abuse survivors hear and the response is often times “it’s not that simple” because it is extremely difficult to leave an abusive relationship. Sometimes because the abuser didn’t always act that way so they are clinging to a hope that the person they fell in love with will come back.

Characters don’t always act in the same way we would because their experiences and environments are fundamentally different