In this thread some person say because the authors show us that this is a bad path, but my character knew this from the very beginning and deliberately went for it for Astarion, up to this point it was possible to choose the appropriate options, and there was not a hint of lust or becoming a vampire, and now suddenly our character says that I want your body or I want to become a vampire (these are the only positive answers) this is not serious, well. In this scene, I don’t recognize my character, who was there the entire game, he was replaced here.
I really don't see this as his bad path. His original creator called it his evil ending and I share that view. To me his bad endings are: getting staked, given to the Gur, becoming a zombie, getting consumed in the ritual, not receiving help with Cazador, becoming a mind flayer, etc.
My motives for helping him were exactly the same as yours.

I also didn't recognise my chara in that moment and had to choose the least offensive line for her, but it still wasn't something I was happy with. All of the responses have unpleasant tones.
I've worked on a pic that summarises the lack of logic and roleplay options in this scene:
https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=939054#Post939054