I would say Shadowheart's scene in the brothel is honestly handled quite well. It's a person who is exploring their sexual boundaries for the first time and I don't see any particular problem with the writing of the scene itself. There are circumstantial problems attached to it like what it becomes if Halsin is involved, and how the contextual reading changes if he's in the party but kept out of it. The other problems are generally more associated with as you said the general presentation of poly, and how the game was advertised as triumphantly including it (big oof moment there for how shallow it is). Like an unfortunate volume of Act 3, their idea of inclusion is half-baked in spite of the way several characters drop hints about their own attractions throughout Act 1. That's not to say they all are or should be poly, but for example Karlach talks like she's at minimum okay with being included in a polyamorous thing if you have another romance active, right up until she isn't and that's even more bewildering to me than why Halsin turned out the way he did. Why bother giving the other new character dialogue baiting that sort of expectation just to cut it off at the knees?