I don't think anyone (except maybe Fuuko depending on how master manipulator she's being) has done anything wrong from a moral stand point yet. It's really unfair to say a couple closeted high school girls are all not decent people are abusive or whatever yet based on this.
In my opinion, realizing your current relationship isn't working even if it's because of "cheating" by making out once isn't a bad thing. Especially because Yuni wasn't even tempted to cheat until she realized she how unhappy she is. The bad thing is not acting on that unhappiness by continuing to date the first person while cheating on them. If Yuni uses this as a springboard to communicate with her girlfriend or to break up with her, then all of this is just normal growing pains for teenagers in their first relationship. I doubt the manga will go that way because drama, but everyone is still good in my book as of this chapter.
Nanase is clearly more in love with volleyball than her girlfriend and there's a cultural reason she thinks that's okay because Japan's model of success is the "man" works 24/7 to support the family and the "woman" waits patiently at home cooking and cleaning. If she ultimately can't make time for her girlfriend such that they're both happy, they should break up and that's okay.
Yuni just wants to be with her girlfriend. She wants to feel valued and for her girlfriend to be considerate of her feelings by showing her gestures of love and appreciation. Because they have terrible communication, it's possible Nanase honestly doesn't realize how bad it's gotten in which case Yuni needs to learn to communicate her needs honestly. If she can't do that or if Nanase doesn't compromise after being told, they should break up and that's okay.
Fuuko is definitely the creepy one with her stalker tendencies, but if we give her the benefit of the doubt and all she's doing is following Yuni around because she's a lonely baby gay who finally found another LGBT friend, then she really just needs to learn some boundaries which is not unusual for a teenager. If she's been intentionally putting a wedge in by targeting Yuni when she's desperate and planning to interrupt their anniversary then she's more of a scumbag. And she needs to learn not to mess with people. Possibly with therapy if this is about her family problems and less about Fuuko herself just wanting a girlfriend. Even then, while she needs behavior correction, she's just a kid and can hopefully grow from here.
I'm looking forward to the train wreck after this where all of my careful considered character analysis goes right out the window for the draaaama. :D
I wish everyone was as appreciative when it comes to enjoying a good manga story as you are.