What Fuuko did is something that any Cheater Will did. She wait, give something that her partner can't give, etc. In another genre of manga what she did Will be seen as "righteous", because it help the protagonist happy. She is bad person, but holy cow she is really good Cheater.
What Nanase did is something that many Cheater person did. She focused to much to something that matter to her to the point she neglect her partner. Living in her own happy world where she and her partner happy without doing anything much.
In Yuni case, she is someone that felt neglected by her partner, so searching for other comfort. Rather than speak her concern and problem to her partner, she decided to cheat on her. And even then she still saw herself as the victim in this relationship. She justified what she did, even though she have power to break the relationship and have a justified a reason, but nope she rather cheat.
Yuni is the main bitch of the year.
