The book is about her 'rivalry' with Kosakai, all fought out fair and square and in the best tradition of good sportsmanship. The 'just' of the title is the 'fair and equal' just, not the 'only' one. It's a sweetly humourous love triangle, with everyone being terribly chivalrous and considerate about everyone else's feelings. The only trouble, from a western point of view, is that there's uhh 'just' no resolution to the situation. Sekine makes the decision not to make a decision, helped out by Kosakai who sends him after Mieko when she rushes off in tears, having concluded that her suit is hopeless.
Nothing happens sexually, of course.
We only have one small indication from the perpetually ironic Kosakai as to how he feels about Sekine. In the end Mieko gets a judo scholarship to study abroad for the Olympics, and the guys are left still rooming together, and probably not having a relationship beyond that of good friends. Mieko wins her gold medal and tells the world, as well as Sekine and Kosakai, that now she's coming back. I'm a fan of Minekura's other, more famous series, Saiyuuki. But no matter.