To be fair, with all those bugs still in the game I wouldn't be surprised if this is just a false romance flag turned on that would only be active if your character is in a relationship with both Shadowheart and Halsin. It still sucks to read it, I don't think Astarion has lines like this where he fantasizes about Halsin. But you don't have to kill Halsin - you can just keep him in the camp and don't take him with you. Then the banter shouldn't come up. Yes, it is in the game, but like a lot of other dialog that you will only see under certain conditions it doesn't mean if you don't see it in your playthrough that it is still "canon" that it happened.
Still, nothing wrong in letting Larian know how this makes people feel. It will take a while but I still plan to romance Shadowheart in a future playthrough, but I will certainly not visit the brothel with her and Halsin....
