The way in which Daenerys answered Margaery made her rethink the offer. Perhaps she was insulted by it...?

"If you do not wish to, please tell me now. I would rather the truth of the situation than a pretty lie, dressed up with smiles to make me feel at ease."

It wasn't every day that she made an offer like this, but then again it wasn't every day that Margaery led with her heart instead of her head.