Man, I hope this doesn't sound insane, but I made an account specifically to vent about this! I didn't know so many people felt the same way! I've been reading through this thread the last couple of days and I really agree with the majority here.
I went into this game completely blind (I knew the basics, of course) and I've had my heart set on Halsin since Act 1. I roleplay pretty conservatively and I thought a noble, kind druid would fit my Tav perfectly.

I'm giving you the perspective of someone who went into this game totally blind - I had NO idea that he was polyamorous and he didn't hint at it at all or give me any kind of "vibes". Matter of fact, I thought he was the type of guy to be super "obsessed" (think about his monologue where he says when he has his heart set on something, he can't forget about it, and such) and I was super excited to start the romance in Act 3. I was flabbergasted when he dropped the bomb on me! I turned down literally everybody else and didn't even engage in mild flirting because I though he would be mad at me! I'd just hit the 100 hour mark on my playthrough and I felt, to put it simply, bamboozled. It was like I'd been played by a fictional guy and I literally had to turn off the game for a while after this because it really stifled my enjoyment. It didn't match my Paladins attitude at all.

Worst of all, when you ask how the relationship is going, he absolutely refuses to properly define anything and doesn't even give you a proper answer? Like, "oh, I feel great" or something. Just a weird, disingenuous monologue about how relationships are a "civilised" term and how the word is foreign to him? I had to roll my eyes at that statement...

I don't have anything against a healthy polyamorous relationship in a game like this, but the way this was handled was too sudden and shoehorned. I agree that this seems either like on-the-nose fanservice or a personal kink being worked into this game because it doesn't fit Act 1 + 2 Halsin at all. I hope Larian sees the reaction that people have about this and at least patches in some kind of compromise-y line for your Tav to say. Felt odd to just agree without adding my Tav's 2 cents. Okay, rant over!