
I agree it is worse betraying a friend, but the fault doesn’t lie ENTIRELY with the man. And Mari has no right to judge, because she is the same. Mari knew he was married, and still decided to be with him. Regardless of whether or not you know someone, you should at least have the decency to stop when you know the person you are with belongs to someone else. Mari and her lover both need to take the blame.

Well, yeah, there are nuances to it, I guess I spoke too hastily. I agree that if you know that the one you want to go out with is married, then maybe you should think it over once more. However, I still feel that most of the blame lies with the other part. They're the ones who're doing the betraying, after all.
I suppose it depends on where you live, and how it all is looked upon. In Korea it's probably not too uncommon with how this whole drama played out, and how women who sleep with married men are viewed.
Mari is obviously not too good of a judge of character, as this kind of thing has happened frequently with her. But since Junyoung knows of this already, it's kind of already within the foundation of their relationship and it sets a level of standard between them. How they act towards each other is already based upon this. That's why it's definitely worse in their case to betray the other like this (although hypothetically here).
Chill, guys! Mari is just teasing him not scolding him, and if we talk about it, she has done something worse, she went out with a married man...