The answer is simply this: they can and should for any reason or none.
If we all believe that the only true right to govern is derived from the consent of the governed then the consent of everyone is required.
You left off a caveat: "except for slaves, of course, their consent is not required."
I might give Southern secession more credence if everyone had been allowed to vote on it. Is it really the consent of the governed if only white males that own property are allowed to vote?
I have no issue with your arguing in favor of limited government and the consent of the governed, I applaud your persistence in doing so, but the CSA is a really lousy anchor to tie your boat to.
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