Well, I think that I am old enough to differentiate. Cheaters and cheaters might be not the same.
Yes, cheaters are bad.
But not all cheating is the same.
We are talking about teenagers and their first love. We are talking about unfinished personalities. We talk about mental unstable minds because of their backgrounds.
So I sum it up:
Nanase: She need someone else than Yuni. They do not fit. Nanase is not able to give Yuni what she wants.
Yuni: Well same as with Nanase. But she can grow/mature with Fuko. Maybe.
Fuko: Absolutly not sure. She can give Yuni what Yuni needs, but would she also mature when with Yuni?
I ship Funi but I also hope that Nanase will get some happiness with someone else who fit her more.
Would be nice if they could build up a friendship in the end.
For Fuko, I do want to see her mature and find peace, seeing how she grew up.