But is it that surprising that Aziraphale, formerly an angel of Heaven, lover of all things earthly, bender of rules, did something that angels are known for not doing?

"Oh, I, I don't know..." But he doesn't really look uncomfortable, with the coy way he smiles, thinking about the idea of actually sharing a dance with the demon. Perhaps an invitation, ironically enough, for their own song and dance as always, if the demon can convince him.