It's Yuni's fault for cheating. Fuuko did nothing wrong. Nanase should get with her volleyball friend because Yuni treats her like shit.
Hope this helps.
That's if you're seeing the cheating as the sole problem but for me, it isn't. Yuni and Nanase had a troubled relationship which led to the cheating. I think it's a domino effect. One action led to another. Yes, Yuni is mostly to blame. To say Fuuko did nothing wrong is also a stretch? She knowingly pursued a lonely girl in a relationship, essentially preying on Yuni's vulnerabilities in order to assert herself in Yuni's life. Nanase, I'd say, treats Yuni like shit too by constantly disregarding her girlfriend and putting her on the bottom of her priority scale. Have we forgotten she forgot their 6 month anniversary? How she barely interacted with her in public and always chose to hang out with her volleyball friends instead of making time for her girlfriend?
Yes Nanase has a reason for that. As the author has so clearly and beautifully depicted, they all have reasons and motivations for their behaviors. It's what makes them so flawed and beautiful.
But I understand some people have tunnel vision when it comes to playing the blame game.