Marriage is none of government’s business. Marriage was formerly, and is still in many places even today, a relationship that fits within the ceremonies and customs of a religion, not of a state; and it should stay that way.
Why? Because the government has no competence to set rules for or govern intimate relationships between persons **; and because it claims a monopoly on the use of force. Is that the kind of foundation you want for your marriage?
Does government have any valid concerns here? Only two: (a) Protecting minor children and other persons from harm, and (b) Providing a court system that may enforce, if called upon, contractual relationships (such as civil unions). These functions have nothing to do with marriage, per se; they apply to everyone regardless of marital status.
Ideally, all relationships commonly called ‘marriages’ are really civil unions. Partners in a civil union may also seek a true marriage, that is, performed by a minister or similar religious figure. Those who are non-religious will happily avoid this unnecessary complication.
** Perhaps priests and rabbis don’t, either; but that’s a different question.
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