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it has to do with the Constitution. Marriage is a defined and broad term in all states. The Constitution provides provisions that all states must honor legal documents from others states such as death certs, marriage license and civil unions.

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Not so. States may refuse to give "full faith and credit" to the legal acts of other states that are offensive to the states' public policy. So the constitutional problem you cite is not really a problem. The equal protection clause is something else, but that already applies nationwide so it does not provide a basis for your worry that a single state can set nationwide marraige policy.