So yes, as I said, if you treat Halsin's thing not as proper romance , but fling or just casual sex - it makes much more sense with all her dialogue in act 2(her denying poly proposals). She is just ok with open relationship(just like Astarion/Karlach) if Tav is ok with that, but remains mono.
I think it comes down to understanding of poly - I've read all the attempt the classify poly relationships and think it comes down to this: it's poly if the partners think it's poly.
I consider "open relationships" a type of poly. Usually used by people who want to be poly but don't want to consider themselves part of a club or who just really, really hate mashing latin and greek roots together.
I'm not gigaexpert on this topic, but isn't this description pretty accurate?: What is the difference between polyamory and open relationships? In an open relationship, you're free to have sex with other people but stay emotionally committed to only one primary partner. In a polyamorous relationship, you're committed to loving multiple partners equally. I understand that its different for everyone, but it applies pretty well to SH and her general behaviour through earlier acts(again she denies any poly proposal even for Karlach and Astarion). I mean that she is just ok with open relationship, but shes not poly and not interested in it.
