I believe, in most cases at least, it's just misunderstanding of SH character overall, and most importantly the timing this whole stuff gets revealed. And it gets revealed after 2 acts of anticipation, 2 acts of you working hard to earn her trust, to know her better, help her to remember her past, to become that person she's been missing her whole life and getting so close, that under your influence she defies evil goddess whom she served all life as she can remember. Then you get long-awaited love scene and the dialog about your future, basically securing your bond. And she specifically says, how she wants to share her life with you. At this point there should not be even a thought about poly relationship, yet here we are. And game actually plays out like it should, unless you have halsin, and only him, as an active member of your party.
Just remember that even when you bring Halsin along and any of this dialogue plays, it has absolutely no impact on the future dialogues. It's completely inconsequential. That alone should tell you enough about how canon and intended it really is - it doesn't change anything. Just shrug it off for now and see it for what it is - it's a videogame and sometimes things just don't work out logically. If you haven't played Dark Urge yet, just know that you'll have to get used to doing that as Dark Urge in act 3 anyway. There are a lot of out of character dialogues regarding Dark Urge from companions I'm afraid.
So she is still the character that wants to be with you for the rest of her life, starting a family on a farm with an infinite amount of pets, with or without her parents around. Even when she randomly flirts with Halsin, or.. isn't always as thoughtful about some Dark Urge reveals.
And lets just hope they'll fix things. And if not, at least we'll hopefully get an extended epilogue that cements our argument that she's devoted to Tav, as that's how she has obviously been written otherwise.
