Minor spoiler, but the game essentially has a forced mini-harem of three main LIs. You can friendzone the other two (I think), but it's not really designed for that as the expected path (Realm 3 for example focuses much more heavily on one of the other main LIs other than Leyala, for instance). Maybe someone that's tried that can chime in though if they disagree with me.
Obviously play as you prefer, just giving you a heads up that you'll miss a lot of content by going monogamous. Particularly because the other two main LIs are pretty awesome too.