I think I know what is the problem here.
People don't make a difference between someone being poly and cheating.
To many it looks the same (from the outside, I suppose), and they hate poly relationships same as they hate cheating.
But cheating and being poly is not the same.
In this manga is obvious that starting relationship Yuni-Nanase was unfulfilling and needed compromises from both sides. Unresolved disbalance between them opened the door to Fuuko to come in and keep the balance on.
But it was made secretly, at first, behind Nanase's back.
In the meantime, Yuni starts to feel something more for Fuuko, even if she still loves Nanase.
Now it's too late for Nanase to pull back Yuni from Fuuko. Symbiosis is already established. And Nanase won't be able to keep her promises to Yuni. She is attention seeker more than Yuni is, and type who is taking everything but giving not much, or giving nothing at all.
Yuni-Nanase relationship needs serious compromise to survive. If Nanase wants club and Yuni, she must accept Fuuko, who fulfills the void between Nanase and Yuni.
There are much more relationships than we know, relationships between two people who needs additional compromises to function normally.
Let's say if we have a loving husband and wife, and husband gets injured so he can't give all of himself to wife, but needs her total care. Should she leave him and find someone to fulfill all her needs?
Should he deny her needs?
They should find a compromise, and probably find the third person (or another couple) to be included in their marriage. But that third person must be respectful and trustful toward both of them. There's nothing wrong to find compromises in relationships of any type.
But it does not matter how many persons are involved, they must be faithful and respectful to each other same as it's only about two persons relationship, who love and respect each other and are faithful and lovingkind .
Everything can be turned dirty or noble, choice is ours.
