Baldur’s Gate 3 is a story-rich, party-based RPG set in the universe of Dungeons & Dragons, where your choices shape a tale of fellowship and betrayal, survival and sacrifice, and the lure of absolute power. (Website)
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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.
Yeah, they're there.
It's just not everyone is into it
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.
You have okay options to play an ace/aro character, you have options to form close bonds with most of your companions without sex for sure. (Was pleasantly surprised that as horny as Karlach is, you have very clear options to set boundaries there and still have that relationship. ) And of course, you don't have to romance anyone, but can still raise their approval and earn their trust/friendship.
Admittedly though, there isn't really any representation from non-player characters, which is unfortunate.