Astarion, 'I have watched too many people I cared about die, or at least I thought they died. Now I am ascended I would outlive you and I would hate that so much, watching you die and being unable to do anything about it. If you choose not to become a vampire I think we would have to part ways as I can't be there for that, not again, never again if I have anything to say about it.'
Makes sense of the breakup if you choose not to become his bride and is what I hear in my mind when I'm playing through this awful conversation.
Currently I just cycle though it as quickly as possible so that I don't have time to think about it as in it's current state it's illogical and completely immersion breaking.
The thing about him calling you 'Pet' is just utterly ridiculous to me. Most of Astarion's dialogue was written by Rooney, he's from ireland, in common with most of the Uk and both northern Ireland and the Republic of Ireland the term 'Pet' is used as an endearment. It doesn't signify anything more than that, I've heard it used towards people more or less all my life, in London its often 'petal' but its more or less the same meaning - no one decided it was some weird type of abuse. In my Uk ears its the same as him saying 'My love' or 'My dear'. It was probably unfortunate that that the word used was one which doesn't travel in other cultures very well but no more than that.