It's not lazy to have the
option of being polyamorous in the game. As someone who is in a poly relationship I'm quite happy to have the option in the game, in fact.
But it is lazy in the sense that poly is an option with
all the romance-capable companions, just as it is lazy that all the romance-capable companions are "player-sixual". It devalues them as characters because they basically go along with whatever the player does romantically without a fuss or consequences (and for anyone who cares, a little text box saying "X didn't like that" isn't a consequence

).