Non sequitur.

No one disputes the right of any citizen to promote their views, whatever the source of their convictions. The point is that notions like "Marriage is purely religious" is fact-free ideology that has no legal resonance. The history of the institution primarily concerns the disposition of property and the production of heirs in which the woman marriage partner was considered chattel, a condition enshrined in the Tenth Commandment:

<<Thou shalt not covet thy neighbour's house, thou shalt not covet thy neighbour's wife, nor his male slave, nor his female slave, nor his ox, nor his ass, nor any thing that is thy neighbour's."