The question was like a riddle to William. The question popped up in his head. If he answered with a lie, what would happen and how would Marion react if he answered honestly.
Wasn't it obvious that William couldn't mark Marion that night? There are already wounds left but Ange makes it clear, that Marion is a human that many creatures can have.
Maybe like Amethyst, or even like Ange who can never be owned by anyone.
And another fact was finally revealed that Marion was not an ordinary human. There was wolf blood in him, so what else? What other secret was there that William couldn't fully believe and it was difficult to pinpoint it.
It wasn't that he didn't want to, even Marion herself seemed to want that. He wanted to be fully owned by William, but how?
"Marion... I have something to explain to you." William grabbed Marion's fingers, then gripped them tightly. "It's not that I don't want to, you must have heard from Ange and Leah what I'm like."