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.
(See also: Late Corporation of the Church of Jesus Christ of Latter-Day Saints v. United States, 1890.)
No man can become a law unto himself under the guise of freedom of religion... Marriage is a religious rite, not a civil right...
"We have the right to regulate practice, not belief." (Reynolds)
Yeah, but they said that criminalizing sodomy was Constitutional too, and then overruled it just 17 years later.