Honestly I am a bit surprised that you, of all people, don't get this.
At least, what I think
BlasKyau is saying is, if one girl is interesting enough, then other girls don't matter.
It's similar to real life: if you fall in love with a girl, the fact that she has a hot best friend / sister / mother doesn't matter anymore, because you are in love with that girl.
Many games don't achieve this. I blame that on flat characters and uninteresting writing. But, although the story is still young, this game does have this potential.
Some games do have deep and interesting characters. Emma from Young Again being a good example, in my opinion. Unfortunately, often the experience is weakened by introducing other girls which are also too interesting to ignore. In Young Again, I have difficulty chosing between Emma and Ruby, which is fine by me, because they acknowledge and accept each other. But then the game also introduced Grace, which I find a bit too interesting to ignore but also detracts from the Emma/Ruby/Anna experience.