The issue with Mizora and later Halsin scenes with Shadowheart and Astarion is that they feel rather tacked on. So any argument that I see around it kinda falls flat.

Both characters throughout their romances (especially near the end) establish that they are only interested in the player character. (rejecting any attempt to open the relationship/breaking up because of it ect.) They both have a past of casual sex and abuse that is masked by flirty/joking behavior but that mask drops as soon as those relationships become serious.

All I see is a hasty decision on the developers part where they felt like giving more 'options/choices' was the goal no matter if it made sense. They picked two companions at random and then just added the Mizora scene as an attempt to justify that decision. (regardless if it ended up contradicting what was said before)

Shadowheart goes from 'I don't wanna share/I'm ending the relationship because you are pursuing someone else' to 'I never wanted a pure swan/I don't care that you are sleeping with someone' out of nowhere.
And with Astarion everything outside of a committed relationship just feels like coercion since he's in a vulnerable state.

In all honesty like the rest of the game, romances feel unfinished/patched together.

It is true that polyamory is a thing in the Wood Elf culture but so is bonding with your partner, especially if that partner is of a short lived race. If they focused on actually developing Halsin as a full companion they might've even succeeded in making something good (poly and mono versions of his character) but to me the way he is now just comes of as a kink fulfillment without any depth. And to add to that, that kink ended up affection other characters in a bad way.