I think Milsonwheels95 means that Halsin will still participate in the orgy if the player's monogamous partner is at camp, and don't even know the orgy is going to happen. I did some quick testing (so I may have missed some variations) and found that the Drow twins did not agree to the group scene if my character's partner wasn't present, so Halsin didn't ask if he could join too.
HOWEVER there was an exception to this. If Astarion was in the party (I was not partnered with him), the Drow twins would invite Astarion. Astarion agreed, and if the player agreed to that, then Halsin asked if he could join. So in that particular circumstance, Halsin does not care if the player cheats on their partner with him. I'm hoping this was just a programming oversight (since in his confession scene he says he wants their consent), but it is something that can happen in game.
