You're really confusing me. It's like you can't make up your mind on it. Like one minute you say it's "kinky and a fling", then another you're like agreeing that it's mono. So I dunno if you're just trolling? But this back and forth is giving me whiplash.
Either you believe she is mono or she isn't. Which is it?
I have 2 interpretations: 1.) She is in "open monogamy" with Tav and it is "sorta" consistent through the story if you only consider Halsin as a fling(He doesn't even like when Tav asks about relationship, the word I mean). 2.) She is full mono with inconsisteny in writing to let all the kinky act 3 staff in. Some companions had to be "shoehorned" into this to justify these activities(Astarion, SH and Karlach were the choice, sadly).
But where does she ever state or even imply that she's in "open monogamy" with Tav? Why would Halsins lack of interest in open relationship only affect her and Astarion (Karlach doesn't actually join in on this I believe)? Do they ever mention Halsin' openness as an excuse for them being into poly? Why does Halsins relationship preference have anything to do with SH or Astarion? Neither of them care in solo Halsin romance and I've never heard of Origin SH or Astarion being into him either. Why is she suddenly into him when she never expresses this throughout the whole rest of the game? Why him? Why is she not okay with Karlach or anyone else?