I mean, it sounds like Halsin had every reason to assume the player had their partner's consent if the player and Astarion were already planning on having an orgy.
The thing is, the player and Astarion weren't planning on having an orgy in advance. The player tries to have a threesome with the twins, the twins say only if there's another person and request Astarion, if he's in the party. Astarion's dialogue makes it clear that they had not previously planned it, and it was spur of the moment. I think assuming the player had already discussed it with their partner would be naive, and Halsin is supposed to be wise.
Considering being wise and caring about the partner's consent are already established for Halsin, I'm inclined to believe him asking to join is a programming oversight, as I mentioned it might be in my earlier post. It's a specific set of variables and could easily have been missed in testing.
I think a lot of the brothel stuff is poorly thought out/implemented, and I try to not let it impact my views of the characters too much. It is in the game though, so unless Larian fixes/changes it, I can't blame people when it does impact their perception of the characters.
