The game is very linear right now, you can't really do the girls' routes at the same time. You have to do Asami's route first, then Yasuka, then Sayuka. Idk if this will ever change in the future, but I doubt it will.Is there a harem ending planned for the game? So far the routes doesn't seem separate from each other, so you have to do all of them (unless I'm doing something wrong. Sakuya's route only unlocked for me after doing all other girls routes). But 2 different characters have already told MC that he will have to choose and at least right now, Asami doesn't seem to like very much the idea of a harem. And that made me a bit concerned.
If the end comes to choosing only one, I don't really know how to feel about that. There is not much point in the game forcing you through all the girls (again, unless I'm being the big dumb) and then have you make a choice at the end.
Having finished the game so far, it's pretty clear it's heading towards a harem direction. In fact it doesn't really look like you'll be able to choose not to have a harem.