And...we should not forget that Fuuko did not answer Yuni why she loves her, they were interrupted by forceful entrance of Nanase.
She literally entered, grabbed Yuni like her possession (and Yuni liked it!) and took her away from Fuuko.
Forceful and possessive, like when just couple of chapters ago we had a scene where Fuuko openly undressed and groped Yuni in front of Nanase? I guess opening a door can be viewed similarly, perhaps. Yuni probably liked that too though, maybe she is just a fickle bitch?
Of course it's very similar, as you noticed. We can see from both examples that Yuni is submissive and enjoys to be taken, swept away (probably something close to "consensual non-consent").
Definitely she won't fight against it, as we saw from before in her interactions with Fuuko (it was "borderline assault"). And it would be, if Yuni is not as she is, consensually submissive without verbal acknowledgment. And Fuuko respected her boundaries, understanding them without speaking verbally with Yuni about them.
That's why Yuni chose Nanase this time, because she was turned on by situation and how Nanase dominated above her and Fuuko.
Probably she likes such an energy in her partner, it moves her.
And probably she feels something close to love for both girls in same time.
Does it make her bad? Not sure, she is just a typical submissive character with poly inclinations (yet not aware of it). She is something like a cat, as her avatar shows already.
You're right. This chapter made me furious for a dozen different reasons, but then again Yuni is exactly as you say. It's in character for her to react like that.
Nanase's deal is all wrong no matter how you look at it. Looking for Yuni in her free time, taking her on dates, displays of affection, sex, they don't come natural to her. She was perf happy with things as they were till now, and she's offering to make sacrifices (not big sacrifices like the club, small sacrifices) to win Yuni back. For Yuni, she'll do stuff she wouldn't do otherwise. But the point is: all those things should not be sacrifices!!!! If spending time with a girl and having sex with her is a big concession you make against your natural drive (which is to go do some more voleyball) and only agree to it to humor her, then you shouldn't be dating her in the first place!
Yuni should realize this... but the author herself said that she purposefully put in Yuni every trait she finds annoying in a woman, so it's no surprise that she made the wrong choice.