Ok I have a few things I want to say, suggest, kick up about, regarding Astarian, with a side helping of a no less important Karlach.
1. I have no idea how the writers even thought Astarion was a suitable contender for polyamory. A huge part of his arc is him trying to escape forced and ultra compulsory sexual slavery and trauma. He's mentally scarred by it, he finds it difficult, if not impossible for most of the game, to say 'no' out of fear of losing the one person that has stood by him. His entire flamboyant character was conceived by Cazador to make him more suitable for what Cazador wanted him to do. You have to work to gain his trust, getting him into bed once is easy, after that if theres no relationship you can forget it.
I've seen the scenes on utube (yes I was curious on my third playthrough), he doesn't really take part, the game says that if you look into his eyes he's 'a million realms away' or 'there's no joy in his eyes', he performs because he knows how to but mentally distances himself from it - its too close to what he spent 200 years being forced to do.
He's doing what he thinks you want him to, turning YOU into his abuser.
Yes I know how to avoid those scenes, I've had several playthroughs and I am a long time gamer and can read a scene reasonably well.
But why on earth would any writer think he's a good candidate for this? It's mind boggling, tone deaf and ultra distasteful - really.
I was just writing a post about this scene, but I guess I'll just chime into your thread to not be redundant.
I think what I hate most with the drow twins scene is that you can't stop it once it's started. Astarion himself says that he wants to try it and assures you that he'll abort if he does not like it. So you go into it trusting that he will say stop (reminder that the scene can only play after you defeated Cazador). But he doesn't abort. And you as his lover see that he goes into service mode where he takes care of everyone but himself and then compartmentalizes out of the situation. But you get NO chance to stop him or the sex scene as soon as you realize he is not ok. You have to let it play out to the end and the drow makes a tasteless comment that he should pay you for it. Even worse – you can not talk to Astarion afterwards. Player agency is taken away during the scene and you can’t even have a talk. It's really badly handled IMO. What was the point of the scene if you can't even react to it?
That said, I think it's totally in character for him to want to try stuff (even sex stuff) now that he is free of Cazador. As he puts it in the graveyard scene - he wants to try everything life has to offer. And the first thing he wants to do is have sex with you. It seems to go well, so he gets cocky. And he tries the next level too quickly and can't handle it. Totally ok writing. But as a player you should be able to drag him out of the situation. So my problem is not that the scene exists, but that the player has no agency in it. I think they should really give you the chance to stop him and then have a talk with him.