A well written rpg tackles roleplay possibilities while keeping the narrative intact. The game should not force motivations on your character and take away character agency for the sake of a certain narrative. As you said yourself, whatever you say, it will direct back to the ultimatum - be my spawn or this relationship is over. And that's totally fine.
I guess my biggest problem with this dialogue is that it wipes away the whole narrative we had up until now when it comes to the romance. It totally negates the pivotal Act 2 confession scene where we can show him that we really care about him, not because of his body or the power he represents as a vampire - him as a person. Otherwise the romance would not progress. I mean, you can even mindmelt with him and show him that you are not lying. But suddenly when we come to the dialogue after the Cazador fight it's "Psyche! We were just in it for the sex and/or the power after all!"
You can see this dissonance in the narrative especially in the scene where you read his thoughts when he commands you to kneel. His thinking of you degrading yourself is a throwback to a dialogue you can have with him after the Araj-incident when you tell him to use all his assets to get what he wants. He says something like "You are right, it's not me degrading myself to manipulate others, it's the ones who fall for it who degrade themselves". The problem is just that this dialogue is a non-romanced one. ONE option in the whole dialogue. But if you follow his romance path this never comes up, he never comes to this conclusion - on the contrary, you actually nudge him towards the revelation that a relationship between equals is a possibility. You even abstain from sex to give him time. Why would this be wiped away just because you let him have his revenge (and what is better revenge than taking all that power from Cazador that he painstakingly worked towards for centuries?)?
I mean, you could argue that he really manipulated us the whole time with his Act 2 confession, but in that case spawn Astarion wouldn't turn out the way he does.
In the end, the game just contradicts its own romance narrative for the sake of telling the story of "Astarion the abuser". I have nothing against Ascended Astarion being an abusive asshat to Tav, but it has to make sense in its own narrative. And it doesn't really. They even made Astarion revert to feeding you his favourite lines again and Tav can't call him out on it. For this to work you have to wipe away the whole romance narrative that has been built up until now.
So, for the sake of the narrative (the WHOLE narrative, not just the "Astarion the abuser" one), please let Tav tell him they did it for him and don't need anything back. That doesn't make Tav naive or stupid (as you said) but is perfectly in line with the whole romance narrative up until now. I can accept that evil hell magicks does something to you, turning you into something really evil, but Tav is still Tav.
I agree with you, honestly! I think at the end of the day I focus a lot more on narrative than the roleplay aspect so it's not in my priorities, but I understand that other people really value that (esp in an RPG game) which is why I think adding an option is def good. I see how the ascended romance contradicts the Act 2 romance but I also think that (hot take coming here) a romanced Tav who helps him ascend has either failed to see the crux of his issue or is ignoring it completely, perhaps because they hold certain views about power themselves, so either a well-intentioned naive person or evil/selfish. I always play fucked up Tavs so this isn't a problem, but I think if you're playing as an evil Tav you've already contradicted your choice in act 2 (maybe Tav was playing the long game, maybe Tav didn't care about sex so it didn't matter to them... up to the player). I agree that a naive Tav who just thinks giving Astarion whatever he wishes will help him hasn't however been inconsistent in their logic. I suck at writing my thoughts so I hope they're clear lol
