You don't view the relationship as transactional, but the character you're speaking to does and they want to make that clear because it's an important part of his story.
I don't think anyone here has a problem with that. But it should be made clear with Astarion's words, not the player's. The dialogue in question only has the following options: 1. "turn me"; 2. "fuck me"; 3 and 4. "let's argue". There's nothing to pick if your character for example doesn't want anything from him and isn't interested in picking arguments. Astarion's personality doesn't need to change if the player is allowed to say "i don't want anything." He'd just continue pushing and it would be in character.
That would be fine. But that's not the only thing being asked for here at all or the only sentiment being expressed.
This is literally the only thing I asked for. Add a positive phrase for Ascended and a negative one for Spawn.
Being a vampire like you is not a positive option. Because it's barter again. Besides, who told you that my character wants to be a vampire or planned it? Exactly. Why should I choose such a narrowly focused answer that I never even thought about. Oh, wait, because the other three are aggressive... well, wow. And we come back to this problem again.
There's already a very negative option if he doesn't ascend. You can tell him you're surprised because you thought he was "just a power hungry mad man."