Shadowheart and Astarion flat out do not want to share in Act 2. The poly thing was forced in Act 3.
That's a very good point, and I see a huge difference too, having watched the videos of Astarion and Shadowheart in Acts 1 and 2 (regarding the “third” in the romance) and comparing that to the video of their agreement to it in Act 3. If you watch those videos in a row, it seems that our companions have Tav digging into their brains with the help of a tadpole to switch some “button” there. Moreover, the somewhat intrusive suggestion of polyamory occurs precisely when Tav and Astarion or Shadowheart already have a serious relationship, the player has had time to become attached and form an opinion about the relationship. This is especially acute in the Shadowheart romance. At least Astarion himself doesn't want any polyamorous relationship with Halsin, the player himself has to push him to do it, if you refuse Halsin right away, there will be no ambiguous banter between him and Astarion. And while it's certainly frustrating to see my favorite character being used for a bear video, it's still okay to say “No, that will never happen in my game, and it doesn't exist in my world”. In Shadowheart's case, unfortunately not all players can do that, and I completely understand their frustration and outrage over it. By the way, Zevran in DAO has the opportunity for polyamory, and he flirts with others, but only until your relationship has become serious, then, unless you yourself have used the opportunity for polyamory, there will be no hints of it, the relationship will be monogamous. Same with Daeran in Pathfinder, the opportunity is there, the opportunity is easily ignored, there is no sense of imposition.
I think BG3 should have also approached this topic more carefully and gently. Halsin should have first asked the couple he wants to join how they feel about polyamorous relationships in general, that would have been polite and reasonable. And only if they are willing to offer them. Also, just as it's unclear why Astarion and Shadowheart specifically? Both are traumatized characters, in Astarion's case it generally looks like he's only agreeing to this because of his affection for Tav. It doesn't seem like a positive representation of a polyamorous relationship. Wyll, for example, would be a much better fit for the role (Gale hardly, the character doesn't really fit), if Wyll happily agreed to a polyamorous relationship with Halsin while giving a speech about how good, free and generally “good” it is, it would look much better than in Astarion's case. Lae'zel, who offers sex to the player before any other companion, takes it very lightly at first (consistent with her upbringing and githyanki culture, but without any hint of trauma or being “taught anything”), is monogamous, and Shadowheart is poly. The only explanation I can see for why Larian decided to make poly specifically with Astarion and Shadowheart - they are the two most popular characters for romance among players, it's hard to find any other explanation for this choice. And this decision most likely came later, so polyamory was crammed into Act 3 without caring about the tactfulness of the execution or the consistency of character development.
