Except that nowhere does it say that Tav is in a relationship with Halsin. Even in the new epilogue. They just live together, sleep together, and that's it. There is not a single declaration of love (I love you), there is no declaration of an official relationship. So the most canonical polyamorous nature of the wood elf is preserved until the end of the game.
If you live together and sleep together you have a relationship, whether you put a label on it or not. That this relationship comes with individual autonomy and independence and without commitments or long term declarations does not make it any less of a relationship. Is it purely physical and about sex, or are there romantic and emotional feelings involved as well? That's up to you do decide. If you play the scenes where he displays his joy of getting your attention, the embraces and the forehead touches, it is quite easy to imagine this relationship as romantic and emotional.
I don't get what you are arguing against here? That it is somehow wrong to imagine your relationship with Halsin as romantic because as a wood elf he is non-monogamous? The two are not mutually exclusive by any means.
