His skepticism at this is of a whole different sort. It's a question of other people's strange habits, and so Arthur's smile doesn't hit his pride. There's nothing unreasonable about not knowing something, and Arthur's answer makes him certain that there's no way he would've guessed.

"But being a magician means something about who you are. People can go from in a relationship to not?" He feels that's a logical assumption, as you can go from not being in a relationship to being in one. Apparently. "It's normal to go from being an apprentice to being a magician, but you can't go back. Besides, what sort of status is that? If you're going to get married, you should just do that."