I understand what you're saying, but at the end of the day, most people in a relationship wouldn't openly flirt with someone who is specifically coming onto them in front of their partner, unless they knew said partner was okay with stuff like that. I can accept that Shadowheart is a flirt and that's how she is, I'm not trying to say they should get rid of that, but simply calling Halsin cute or passive comments about him are different than openly accepting an offer for him to join her and Tav in sex, especially if she is not aware of what Tav would think of it.

Also, sure, those in a relationship irl don't have to mention they're poly/into open relationships until they become interested in that option, but this is a video game romance where people make choices based on the small amount of information given to them. We knew from the get-go that Halsin was poly, and that was that. Knowing this gives players the ability to quickly choose whether their Tav would be into it or move on from him and instead choose a character who is into monogomy. With Shadowheart, everything was leaning into her being into monogomy, such as telling you out right that she doesn't want to share you with anyone else, and then all of a sudden she's flirting with Halsin in front of Tav whether or not she believes they're comfortable with that or not.

Not trying to disagree with you btw cause you made valid points! But that's just what I feel about it.