I'll do the hot take since I haven't commented in a while. I think what Yuni is doing is something 'normal'. Of course, cheating is bad, stringing along Natsume is bad and she should just end things. That's not normal and is bad. The 'normal' I mean is: we saw Yuni was unhappy with Natsume, Fuuko was able to fill her void, and so now she's fallen out of love with Natsume. That's the 'normal' I mean: to leave an unhappy situation. The only real scummy part is the cheating, so yeah, she should just end it.
At this point, beyond the blatant cheating, I think the bigger issue now is the unusual relationship between Fuuko and Yuni. They're not lovers. It's fair to say Yuni really does hate Fuuko. Before, Fuuko was upset that she wasn't Yuni's #1, but now she's fine being #2 since she sees herself as Yuni's "mom". So, Fuuko probably isn't looking to be GF, but just wants to always be at Yuni's side. Ironically, this is exactly what Yuni wants: someone who will always put her first, is open with their feelings, and will never leave her. Maybe not her "GF" exactly, but a close partner. I guess the big question now is, if Yuni wants Fuuko to be GF in near-future, would Fuuko actually accept that? Since she's fulfilled by being 'mama'.
Obligatory paragraph for Natsume. Yeah yeah, she's the victim in almost all the scenarios. It is a bit difficult to feel sympathy though. It's clearer now what Yuni wants, what Fuuko wants, but we don't know much what Natsume wants exactly. Does she just wanna focus on sports? Does she actually want a GF? Why get one if she wasn't going to pay attention to her/afraid of the attention (regardless of her past)? Or... is she just looking for an ardent fan that will always support her? Because if so, club friend Yuki fulfills that just fine. IMO, she doesn't seem all too shaken up by Yuni's cheating. I hope more chapters focus on Natsume, because there isn't much to over-analyze on her xD. Through no fault of hers, 'too little too late' is apt here. It took cheating for her to realize "darn, should give more attention to my gf" since there was no indication that she saw issues with her past behavior. Lesson for the next time I suppose. Like, even if she forgives Yuni again and gives up EVERYTHING for their relationship, I don't think there's any recovering it. Yuni is GONE.