Idk why people hate Nanase so much, i get that she may not pay attention to her gf as much as she does for her volleyball club but this what she likes to do, she did train so hard to be there where she is right now, if I was Yuni I'd try to understand that this what she likes to do or if she is lonely and needs attention why don't she talk it out with her, it's not that hard right?
It's completely valid for Nanase to focus on her club. The problem is that she and her gf have very different needs and expectations from this relationship (not helped by Yuni not talking, as you pointed out, but even then, how dense can she be not to realize her partner definitely isn't satisfied with the current situation) or, to go further, the fact she would even go out with someone only to avoid being intimate with them while simultaneously being more intimate with her teammates. It's very hard not to believe she's just stringing Yuni along, even if unintentionally.