Very late to the discussion. Like other people, I also think the romance is written in a confusing way. From my subjective point of view, it is not clear if is meant to be a fling (as in ethically non-monogamous) or poly, as in a relationship with more than 2 people. I think that since non-monogamous is not as common as monogamous, the dialog for Halsin should have taken that into account and show better where he stands in this. Communication and talk to make up for not being "standard".

The way I see it, even if Halsin is not a companion until you do his quest in Act 2, you do know Halsin since you rescue him in the goblin camp. And you can start flirting at the party after fighting the goblin. So the party is the first place where Halsin should show/say something that hints a his preferences poly/non monogamous. And to be fair, not only Halsin, but Shadowheart and Astarion too if they are meant to be ok with this or even like Halsin. Astarion commenting that Halsin is hot or experienced might be a hint that he has an interest in Halsin. Or even a comment that they have talked about. And Halsin being openly sexual with another NPC would be a good way to show early that side of him. Then in act 2, the same way that your character starts getting closer to Halsin, comments or scenes showing something similar with Astarion or Shadowheart might show something like that. Here, the OP should be allowed to tell Astarion or Shadowheart that they don't want poly and want to be exclusive. But if not, maybe let Astarion or Shadowheart bring the topic themselves. So by the time Halsin brings the possibility, the MC already knows if their lovers would be ok.

And in order to make sense, maybe Halsin shouldn't be the only one commenting that Astarion/SH joins, but they showing attraction or connection. I mean, I can imagine Astarion talking with Halsin about the time Halsin was a drow slave and how Halsin dealt with that in order to overcome his own trauma or try to.

But if they want to go with the "it is only sex" route, maybe they should get rid of all "my love" and other comments that show a deeper connection. The "I don't do relationships" while calling you "my love" and all the sweet talk don't work well with the non-committed sex. IMO they should choose if it is poly or simply sex without strings and then modify all dialog to fit that. Also, if they pick "poly" with 3 people in a relationship, then the ending should reflect that in a scene that shows the 3. They can't reuse the same ending scenes. To put an example, if in a relationship with Astarion and OC, when Astarion runs in flames, both Halsin and OC should run after him. And the final talk about orphans and what to do after should involve the 3.

But they should add scenes like the one about Laezel and SH fighting with the Astarion, Shadowheart, and Halsin involved to make the poly work instead of just popping out of nowhere. As for the drow twins, I think that it shouldn't be the only option for the poly to have sex together. If poly, a threesome between Astarion/SH, Halsin, and OC should be possible without having to use the drow twins. Specially for Astarion, it makes a lot more sense that he tried a threesome with someone he trusted first (or even only).

As it is Halsin romance/fling doesn't quite work for me.