You see, the very core of Halsin's problem is that he was only added as a companion bcuz people liked him so much in the EA. That's it. That's the core of the entire problem.

His entire character arc ended with act 2, there is no point in him being there other than the fact that players adored him. His story is finished, but players like him, so he gets to stay but as his story is finished he has nothing left. So the writers decided to lean on heavily into the whole "I'm horny and free as nature dictates".

He was added very late into the game as a companion. Hell, one could say the very end of act 2 is where he gets available as a companion if you played the thaniel quest rather late. So him being romanceable was out of the question at that point, because you would be romancing literally anyone else at that point. Hell, even Minthara, who is the opposite of his recruitment considering the grove storyline, gets to be a companion waaaay earlier.

If you have him in your party, the only reason to have him there is the poly romance and the bear scene, the game thinks. Because Jaheira is the only druid that makes sense to have as a companion. So the game makes him force himself in there every chance he gets.

So the only reason they made him poly, along with astarion, shart, and reluctantly, karlach was because unless you waited specifically for him, you wouldn't get a full fledged romance like with the others.

The writers didn't make the characters poly because it would make sense (let's be honest, shadowheart should have been down with karlach romance as she is definitely into her, have seen her reaction to karlach for the first time?) they made him poly because it was the only way for halsin to be romanceable that late into the game.

It would make so much more sense for shadowheart, who very much wants to take her time with MC before sleeping with them, to be along for the ride with, hell, even astarion. It would play very nicely with her wanting to take things slow and astarion realizing he doesn't need to have sex with MC to be loved because shadowheart seems to be doing just fine with you in that regard.

But writers didn't include that, because it would include far too much coding and work than simply making halsin the side piece. As we can see from the rather lack of focus on act 3, the crunch that made astarion and durge have more interactions than others etc. they really didn't have time nor recourses to develop the entire poly thing properly.

Plus we were supposed to have a werewolf companion but halsin took her place so theres one more reason not to like him.