Yes, you won't be able to play each of the routes simultaneously on a single save. It's a deliberate choice on our end. Each of the main routes are about Keisuke's romance with a girl of player's choice. We don't have plans for harem stories. While the early parts of the game allows you to play scenes from each route and get a feel and tone of route's characters, after making your choice to commit to relationship, other routes will remain locked. The girls from each of these routes are still a part of your route and optional events (crossover scenes with other girls, special scenes, MC scenes with events focused on main character's slice of life or RM events which are focused more on learning more about the lore with Kei's roommate Daichi), but the main story will always be focused on a romance with a girl you picked