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 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.
Precisely. And if the Halsin thing is to be considered as no more than a fling, then three things need to happen: 1. Allow you to ask for the same regarding other companions 2. Make it clear as day that this is what it is intended to be seen as 3. Explain why she is okay with it in the first place, so that she can develop past her dialogue in act 2 where she makes it clear she wouldn't be up for that
They did none of the above. Instead, they wrote that it is "bold to ask", something that was not the case in all 5 scenarios regarding the same question in act 2.
Until rewrites, it's nothing more than an inconsistency that ignores the rest of her dialogues ever happened.