That may be so, but marriage was handled by the church during the greater part of the period after 500 A.D.
America doesn’t have a government church.
It was not unusual for a government religion to make marriage law.
It is still the law, imposed on the subjects and in fact is quite common in the middle East and other parts of the world I assume.
If you want Christian marriage law to endure in America, then you have to vote for it, because if you just say “religion”, or “church”, there are gay churches, Muslim churches, Muslim churches, Satan churches, and any kind of world church and future cult or religion that anyone wants.
Besides, you ignore reality, which laws will apply at the federal level, for federal agencies and the military?