I agree with this as well, but ultimately nothing is going to change considering the unintentional implications of restricting poly to the evil paths. It won't send a good message, that's for sure.
Honestly, this is only an issue because of Halsin's late addition as a romance, it's not like they approached this topic with the intention of giving people a good poly representation. It simply was the easiest way to make the Halsin romance option available for as many players as possible without losing their main romance and having to break up with them.
As for Shadowheart and Astarion, they were clearly only chosen to be fine with it because they are the most popular female and male romance options, regardless of any inconsistencies the previous writing creates. The same would've happened if they chose other origins to be fine with this, as none of them were written as poly from the start.
I could also see someone like Karlach being okay with poly, but I think that similarly, like with Astarion, she has some baggage (that being her years of loneliness) that might make people uncomfortable with the idea (and I repeat: that's regardless of whether or not people are right in having that concern)
She is an option for poly, actually. She'll give Tav her consent to be with Halsin. But she gives off the same vibes Astarion does, if not even more so, and from what I've seen in the fandom, most people believe she only accepts it because she's about to die and doesn't want to disappoint Tav.
