The reality is that 'obey' was in the marriage service, but I do not think that this passge is evidence that Mr Knightley literally believed all wives should obey their husbands unquestioningly. He may have done, though I doubt it. He seems far too sane a person and far too intelligent an observer of other people to think this was either realistic or desirable. And the fact that he is joking about it here reinforces that, for me.