Also, what I noticed is that the main gripe people have about Yuni is that she never tried to communicate her "loneliness" to Nanase. And so, it's also her fault if she's been neglected. Commnunication, blah, blah blah...
But people are judging this from a western point of view, where people are used to be told to lay things on the table and discuss the issues.
The way Japanese people interact is different and one of the things I noticed is that they dislike direct confrontation, or criticizing others directly. There's this whole cultural thing about "reading the room" or "disturbing the harmony" that prevents direct communication and expects people to take hints on what others are thinking and avoid making a fuss or putting others in a difficult position. Not to say that some Japanese people don't do it, but it's considered rude.
The whole situation stems from the fact Nanase's unable to read and respond to Yuni's emotions and needs, and for that reason, their relationship's falling apart. And the fact Yuni doesn't confront her is because IT'S NOT SOMETHING YOU DO EASILY IN THEIR CULTURE. There needs to be a real crisis for them to speak their mind, like in the latest chapter. Yuni's at the end of her rope. She loves Nanase, but she finds in Fuuko everything she lacking in her own relationship and she's at an impasse.
On the other hand, Fuuko's really good at it. She can read Yuni and Nanase easily. She can navigate the whole thing without a hitch, so she's able to poke holes in the relationship easily.
Anyway, my point is, trying to judge these characters' behavior from a western point of view is completely different than how Japanese readers perceive it. Even they think "cheating is bad" of course, but to them, the whole situation doesn't seem as implausible or as "easily solved" as some readers here seem to think.
