When you think about it, all this poorly implemented poly stuff between companions is only there because Halsin was included as a party member. We wouldn't have had it in the game otherwise. Everything is accomodated for him. Astarion and Shadowheart dialogues are added only because of him. They themselves never mention being poly or express wanting to try things out with other companions and you (only Astarion, when asking you to choose between him and SH or Karlach, says he would have been ok with poly under different circumstances, but it seems like it's yet another of his hedonistic masks and he's just saying it to make himself look good and shift the "blame" on the others for not being proper candidates for an arrangement, just to keep you for himself).
As cringy as the twins are, it's clear this is only sex and nothing more, and it's something you do together. With Halsin, he's there with you from act 1, watching you and your LI and then tries to insert himself in-between when things are getting serious. It's creepy. He even calls you "love" and acts like he's really infatuated with you, while SH and Astarion think there are no romantic feelings involved.

Honestly, I'd rather it was never put in the game, than added in such a terrible and insensitive way.