I think it's a solid miss that Astarion and Shadowheart don't have an option to say " no " when the player asks about entering a relationship with Halsin. There are multiple options on what to say, all of them leading to the same outcome which, in my opinion, breaks immersion as it only an illusion of choice and has no real consequences. Karlach has the option "Forget I said anything " which I think drops the matter but I'm not sure.