Let's compare a little
NanasexYuni
Are girlfriend for sometimes, don't go on date, don't kiss everytime they see each other, don't have sex... well what do they di really ?
FuukoxYuni
Have some dates, have kiss plainty of time every panel they are together. Have sex, do see each other a lot, talk, and so on
Fuuko maybe was ok at the start to be the second woman. But at time goes on, i think here feeling may have grow and now she don't want to be second.
She has given time to Yuni telling here that she would choose her if she let Fuuko be her one and only.