The purity path, and the choice point that sets you down them, happen after the dark path is concluded. At that point, it will be very clear that you have to pick one girl to pursue and see her path through. IIRC, Selebus has mentioned that once that path is concluded, the game will take you pack to the choice point so you can pick another girl. I can't recall if he said there would be a "true" ending once all the purity endings are seen.
A given purity ending is likely going to end very badly for some of the other (not chosen) characters.
And before we get to that decision point, we can expect all 35 main and side girls to be put through an absolute wringer, because, well, the game is what it is.