I mean why would you even argue... She just DENIES any poly proposal attempt in act 2. I just don't get it, it's not headcannon or anything, it's what she does. And no, it's not because she "cares" about others being mono, she denies Karlach AND Astarion aswell. Again AT BEST you can argue that she is ok with an open relationship(and treat Halsin no more than a fling). Or it's just inconsistency to let Halsin be the way he is right now in the game.
