Please bear in mind that I am not against characters being aligned with other preferences other than mine, I am not throwing jabs at anyone, on the contrary.
There are certainly some things in act 3 that suffers from a bit of a rush and the late decision to include a character with a polyamorous romance path that didn't merge seamlessly.
But, the thing with relationships is that there are always could-have-been moments that just never happened because it wasn't the right time. In act 1 and 2 she is also still having a personal conflict between her feelings for Tav and her Sharran ambitions, so she might just be looking for a convenient excuse to convince herself that it's never going to work and bow out.
In all of the talks between the characters and multiple patches/fixes/content updates, it would be much easier if she would say something "Love is like a garden and there is a plenty of space for many flowers, if you know what I mean" sometime in act 1. But it's not, she literally says otherwise.
Anyway, my point is the situation is avoidable. While I understand your frustration, if you get upset just by knowing about something that can happen but can be avoided - you may be a little too emotionally invested. It's still just a computer game, it is supposed to be fun first and foremost. Everything that happens was brainstormed and written by people, to make a fun and engaging game, and part of that is having a billion different ways things can play out. If you get upset, consider doing something else entirely or taking a route that does not expose you to the situation - e.g. don't recruit Halsin at all or just leave him rotting in camp until Orin nicks him...
It is avoidable, it's true, and I have avoided it (except for that godawful banter that is playing regardless). But I still can't get rid of the feeling, that it is her character, and no matter how much you will avoid it, it's there, and it taints the route for me, and I would still perceive that as an emotional affair on my Tav, this is really unprecedentent thing for me. The only way I could alter her story and her character, if I would play as her OC protagonist, because that is the valid option, but that would be in the cold, hard, Tav-less world. And yeah, I know how ridiculous this is, being worked up about emotional affair by the fictional character, that is practically doesn't have any real emotions. But that's BG3, based on DnD, it's immersive CRPG experience, a nice place to escape from life sometimes, and it's only natural that I have feelings about characters and the story, and got frustrated. Of course it's not the end of the world, I can deal with being taunted online or being upset with the game experience, there are by far worse things happens in life.
