Well, what else is there to say? Nanase probably made up her own mind when Yuki said that. Her club is more important to her. And her new girlfriend can be right there with her, too. Yuni and Fuuko can be with each other, without a care about Nanase anymore. I don't know if Fuuko will be serious with Yuni, but at least Yuni can be with someone who remembers her existence. I don't defend her cheating, but it's a well deserved consequence for Miss. I-dont-remember-my-girlfriend-exists. She can choose the club. That's fine. It's her personal decision. But end the relationship to set Yuni free so she can be with whoever she wants. Yuni isn't Nanase's pet that she only plays with when she's bored.
Also, the premise of "give up your club or your girlfriend" is quite... nonsensical to me. Professional sport players still have lovers and families. It's just another reason that shows Nanase is terrible and was never really serious with this relationship to begin with. If we just flip Nanase's gender, she'd be getting a lot more wrath than this.