It was landmark U.S. Supreme Court precedent Reynolds v. United States in 1878 that made “separation of church and state” a dubiously legitimate point of case law, but more importantly; it confirmed the Constitutionality in statutory regulation of marriage practices.
Article V
The Congress, whenever two thirds of both houses shall deem it necessary, shall propose amendments to this Constitution, or, on the application of the legislatures of two thirds of the several states, shall call a convention for proposing amendments, which, in either case, shall be valid to all intents and purposes, as part of this Constitution, when ratified by the legislatures of three fourths of the several states, or by conventions in three fourths thereof, as the one or the other mode of ratification may be proposed by the Congress;...
Socially, the institution serves the public peace by granting monogamy a monopoly status for reproductive arrangements, disadvantaging permanently with the umbrage of the law the various other arrangements you list on your profile page. This the Congress is allowed to do, to invade the presumption of liberty in all things memorialized by the Ninth Amendment, if they follow due process, for the manifest good and overriding public interest.
On this thread you wrote: In reply to that, I say marriage is a public act that requires a license[.]
On your homepage here, your wrote: I say the federal and state governments have no Constitutional authority to be in the marriage business at all, except where each individual has a biological responsibility for any offspring they produce. [Bolding mine in both cases].
Both of these statements are yours. Do you agree with them both? (Hint: it's a yes-or-no answer).