They are all wrong (the three of them, nanase, fuuko, and yuni), but it's totally obvious that the yuni and nanase thing isn't going to work. Yuni is looking for love and understanding, but with Nanase she will never find it.
Fuuko is the perfect girl for Yuni, she gave her everything that Nanase couldn't give her. The maximum advance that the incoherent woman made was telling her friend about the relationship they have.
And that is the only possible ending in which everyone is happy. Yuni and Fuuko together, without cheating (Yuni would no longer have reasons to cheat, she having what she wanted), and Nanase will be with her most precious love: the volleyball club.