She got fed up, at least partly because as a tiefling herself horns and glowy eyes aren't anything to complain about and she was starting to take his comments a bit personally.
Did you get any [Tiefling] lines about this when talking to him? I very deliberately made a dragonborn for his romance because I wanted to see if there would be a special option when discussing his transformation. And there was!
There was the option in that dialogue to suggest spending the night together, but that felt too fast so we just said goodnight.
I'm not sure he'll actually agree at that point (I picked the same option you did). Someone should test this. For science.
