So, how many of you like "sharing" in a romantic path?
Romance is passion, dedication and commitment to a specific other. If you romance someone, they aren't "a" friend, they are "your" friend, as in possessive and vice versa. It's very distinctive from casual or open relationships, something that this game also heavily distinguishes through choices, code, branching paths, love heart vs lust fire icon, etc.
Of course, thats all semantic and you can also romance several other at once, but its rarely done, simply because it undermines the forementioned focus to put all your affection on a single specific individual. If you romance not one but two persons, whats stopping you from romancing three, four and more? If you romance everyone, you romane no one, since there is no distinguishment anymore, the romance become valueless..
In practice, we only have it with Ian, Lena and Holly. Currently it works, but I question if they can uphold to romance each other equally. All three are different persons, so by definition its not possible for each of them to like the two others exactly the same.