Originally Posted by Seho
Again, he makes it clear from the start, just poly!
He is a 350 years old Druid! The trauma? He was young when it happened.
He is one with nature and meditates, he even talks to his God!
He says it's ok now. He doesn't deceive us! He's just a poly and we can break up if we don't want to.
He is surrounded by monogamy and knows it, he doesn't want a mono thing.
He doesn't give any wrong signals, you should go with him, but he won't be faithful, physically.
What he allows you, he allows himself.
He is a 350 years old Druid, he knows what he wants and he doesn't constantly impose himself, they exaggerate...
They want to manipulate him, force him to try, but he doesn't want it.
They have enough mono chars and they don't want poly. Halsin is the only poly character and that's a good thing!
He doesn't make it clear from the start. He makes it clear from the start of act 3. That's 67% into the game. And for some companions, 95%, because act 3 focuses on the last 5% of their story and everything else is about Jaheira and Minsc. 5% of dialogue for origin companions are in act 3 (numbers pulled out of my behind, but that's what it feels like).

You also wrote down part of the problem yourself. Halsin is written to be the only poly character. So why are Astarion and Shadowheart retconned to be fine with him? Not with anyone else, not even each other; just Halsin. Two of the most vulnerable companions (at that point in their romance) who have shown no interest in going poly suddenly thirst for him. And you can bully others like Gale into it as well, apparently. Because writing a character as poly and having him be the only one isn't what they did.

There are so many negative romance and non-romance outcomes for origin companions that in my opinion this should've been one too, if they so desperately wanted him in this state. You even consider the idea of going poly, the others (any of them) break up with you. That's by far the most realistic outcome, considering they're all rather vulnerable by the time you get to act 3 and you've already gone through 95% of their story and romance. But they catered to thirsty fanboys/girls, so retconning it is.