The "Bad" ending is just outright awful. The only way for you to stay together is to become his spawn. There is no way of persuading him to make you a true vampire or to stay as you are (warmblooded).
To be fair, I think it makes perfect sense the bad ending is... well, bad. That's the consequence of you choices and his whole story: you let Astarion kill 7000 people and become an evil monster, even more power-hungry that before. That's the only logical thing that he wants to control his lover and not make them another vampire as powerful as he is now. Because, you know... vampire lords prefer to have servants over competitors. "Trust me, it doesn't happen".
I'm actually surprised he lets Tav decide if they want to become a spawn instead of just do this forcefully.
