Excellent question!
There will be non-plot breaking ways maintain relationships with all the girls. Some of them will be pseudo-haremish (some girls may know about others) while in other cases you'll just be quietly sneaking around. I have no plans to have a large harem type ending where all girls know about all the others (that seems too daunting a task to write convincingly in a way that doesn't come off as straight porn fantasy). But I'll never put you between the rock and the hard place where you have to choose Girl A over Girl B.
There may be instances where you have to pick Girl A or B or C, for a particular event, but that choice will not end any of your other relationships. Like it's prom night and you have to choose between Cassie, Jane or Amanda. Whoever you pick, it won't destroy your relationship with the other two girls. You'll just play that event with the girl of your choice and then the story moves onwards.
As a player, I hate exclusivity in a game, so as a developer, I can't in good conscious include it in my game.
Ending a relationship is done completely at your discretion by choosing the black colored choices. Otherwise, it's game on!