We probably are. Again I came to thread in state of confusion - "why has this thread gotten so long"
She's not threatened by the twins or Halsin. Others have mentioned Minthara but I've never had her in the party.
I guess it's my reading as someone who is poly. If a partner takes an interest in someone who is strictly monogamous that is a threat to the relationship. Halsin - who doesn't really do 'relationships' - isn't a threat. Neither are sex workers.
So the pattern seems pretty clear to me - no to sharing with people who don't like to share, yes to sharing with people who like to share. It makes much more sense than reading monogamy backwards into the previous conversations.
Poly is a type of relationship, first of all. You of all of us here should know. Which means Halsin, as a supposed representation of poly, isn't even actually poly. Secondly, it is not yet established that Halsin doesn't do relationships at the point of his proposal. The proposal is no different than the one you make regarding any other companion. No, off screen is not an argument, I think we've established that and as someone who asks for proof on the regular, I hope you acknowledge that too.
Additionally, you never even get the option to make a poly proposal regarding other companions. If Shadowheart would be up for poly, then she would accept your suggestion - after all, you ask for the exact same thing as you ask with Halsin, and we established above that he's no less a "threat". Then, you would return to the other party (Wyll, Karlach, Lae'zel, Astarion or Gale) and make the proposal to them too. But that is not how it happens. How it happens is that Shadowheart is not interested in poly whatsoever. Because she's mono. As is clear through the dialogue.
And yes, you do read things very differently, if you think "In truth, I don't think I'd want to be your spare lover. I'd always want more of you than you'd have to spare. Better perhaps to bow out with dignity." implies "clear interest in having others in the relationship."
