Civil Unions can be between two dogs as long as it satisfies a Government legal definition for legal purposes. The twain should never meet.
A Married man and woman should have both a marriage and a civil union satisfying both God and the Government. IMHO.
The problem with the religious marriage theory is that the parties to this contract undertake certain obligations to one another that human nature inclines them to default on unless the counterparty has an enforcement procedure or, failing that, a way to recover damages.
Just as with all other contracts, this requires the power of the ruler to enforce.
True marriage requires no divorce, criminalization of adultery, and laws supporting traditional gender roles (higher pay for men, alimony).
Almost no one supports any of these pillars of true marriage, in fact, all laws enabling true marriage have been repealed. So, no matter what anyone says, almost no one supports true marriage,