Originally Posted by Tharrow
Yeah, I get that. It would need to be an unskippable line, too! Something he says at the start of a dialogue tree. Otherwise it doesn't really help guide players with the aim of giving them a clear picture of his romance style in Act 3, to help them make choices early on. It would be a little strange imo so I understand why this currently doesn't exist. Like imagine having "I am a free love type of guy" be one of the first things a dude says to you before you indicate any interest and before he has an interest in hooking up with you because he's got so much else going on -- I imagine it will be difficult to write it without it being skeevy. But because of it being a video game, maybe realism and part of his characterization needs to be sacrificed to clarity of player options.
Will reply to this but (part of) your larger post as well. Part of the issue is that Halsin indeed gives no indication to the player that he's poly. In fact, it feels like his romance trigger is "PC exists and didn't stone him to death with the goblin kids". I made sure not to pick any kind of flirting options, though I didn't blatantly insult him either, and he still wanted me in act 3. I'd assume the cause is that he's not a character you walk around with for most of the game up to that point, so you don't interact with him as much and thus don't build up any approval, therefore they had to make his romance trigger more broad or it'd be too easy to miss.

While I agree there could've been room for poly for all companions based on player dialogue choices, I assume there's some reasons they hadn't. It's less popular of a mindset, it would require a lot more work in addition to rewriting in some cases, and it might mess with 'in romance' triggers between PC and companions, though I admit those are unfortunately quite lacking, even in act 3 where the pursued romances are meant to shine. That said, the game does feel quite poly in act 1, since you can sleep around rather freely and still pursue the romance you wanted later on. It's not until act 2 that most companions will make you choose.

One of the issues with Halsin's poly approach to me isn't that Halsin is poly, but that it required them to come up with two outcomes. Either force it upon more companions, or have romancing Halsin have a negative ending with any other companion you romanced up to that point. They ended up forcing it on Astarion and Shadowheart, with others having options to be bullied into it from what I've heard. Neither of those work, in my opinion. Astarion starts the game off with using sex as a way to get you to fall for him for protection, then falls for you and when you break through his defensive wall he becomes dependent on you. Not the type of person I'd see sharing all of a sudden, which he doesn't in act 2 with other companions either. Can't say much more about his romance since I haven't played through it yet. Shadowheart avoids advances for physical contact for most of the game (with the exception of the kiss), biding her time for a good moment. She will break up with you in act 2 if you even so much as kiss another companion (poor Wyll) and becomes arguably the most emotionally attached companion out of all of them depending on your choices at the end of act 2. She also becomes fairly insecure, wondering multiple times if and why you want to be with her. She is not written to be the type that's okay with poly either; at least the Selune path.

So none of the characters have been logically written to be poly, including the one the others were retconned for in the first place - Halsin himself. Of course that doesn't mean they can't be, but then people will criticize the writing for not remotely fitting the narrative and thus ruining their immersion. Sort of reminds me of the good old ctrl+alt+del webcomic and the infamous miscarriage saga that ruined its reputation (at the time, anyhow) since while it was a dangerously brave topic to cover, people found it didn't fit the narrative whatsoever. Of course, this isn't even remotely as impactful, I'm just bad at comparisons.

The other issue for me and the one I and I assume others call toxic is a simple trigger that happens even if you decline Halsin's advances, where he will then still try to force himself into the relationship and your romanced companion (Shadowheart in this case, but I'd assume this happens with Astarion as well) will flirt with him and disapprove if you say no. That, regardless of what else (if anything) they change, needs to go.