So Astarion is comfortable saying he won't share the player with Shadowheart, Lae'zel, Wyll, Karlach, Minthara, or Gale, is comfortable saying he isn't ready at the Drow brothel if this is done before the Cazador confrontation, is willing to break up with the player if they have sex with Mizora without talking to him first... but when it's Halsin, magically, he becomes a horribly weak and pathetic people pleaser who could NEVER EVER say no to changes in their romantic relationship? That's somehow more probable than the idea that he is okay with Halsin because he has experience with poly relationships (which he explicitly GIVES as his reason) and which is in contrast to Shadowheart and others, who he says would inevitably end badly because they've never done this before?