Spoilers for Act 3 stuff. Prewarning because spoiler tags don't work on some mobile apps.
Act 3 Romance
Shadowheart feels very liberated once you've spared the Nightsong, and Halsin is very much a free love character. There's even a scene where the PC and Shadowheart can hire a pair of sex workers and invite Halsin to join.
Well therein lies the answer: not everyone is into it. I have to admit that it's odd considering Ed Greenwood's very meticulous (and often problematic) coverage of sex and sexual positivity in the Forgotten Realms lore, but poly isn't everyone's thing. Maybe I'll be proven wrong in the future but my understanding, and current consensus among my very progressive crowd, is that poly is not on the same level as sexuality or gender representation. BG3 needs ace/aro representation first.
It's weird that Gale isn't poly because he has such a hard on for Mystra.
I can kind of get why laezel and shadow heart dont want to be metamours.
I would like to see more positive poly representation in media. I think all there is is like.. a few minor parts in NK jemisen's books, maybe sense8. More realistic to me poly that's independent relationships. Where it's treated like how you treat your friend having other friends, and not subject to unchecked jealousy and insecurity.