I don't have a problem with Halsin being poly. But imo being poly is not the same as dropping someone as soon as you're not interested anymore. And that's the problem I had with it.
As someone who creates characters, thinks of a back-story and gets pretty involved in them, this relationship was just painful. I thought Halsin would be the perfect match because he seemed like a kind-hearted, wise and mature man. Imagine my surprise when my character sacrificed herself to become a mind-flayer and this man just told her to go into hiding. He would go help refugee children and no, my character couldn't come with him because that would scare the kids.
Yes, it's great that he's going to help those children and no, I wouldn't expect him to still want to be romantically involved, but some support would have been nice. This was such a downer and not at all what I expected. It just seemed so cold and out of character for him.
I even went back and replayed the entire ending and chose to go with Karlach and Wyll to Avernus. If you do that you don't even get an ending with Halsin at all. Like he never even existed in your life.