At worst they could just specify that Halsin is not even a relationship, but just fling, casual sex source and SH/Astarion/Karlach don't feel "threatened" in their relationship with Tav.
Astarion says exactly this when you bring it up to him, so at least in his case that is what the game assumes.
Im convinced this is how Halsin "relationship" all of them see. This way at least it's not out of character for them. Still the point about banter as valid as ever and so is drow-twins encounter.
