According to this
The first recorded evidence of marriage ceremonies uniting one woman and one man dates from about 2350 B.C., in Mesopotamia. Over the next several hundred years, marriage evolved into a widespread institution embraced by the ancient Hebrews, Greeks, and Romans. But back then, marriage had little to do with love or with religion. When did religion become involved?
As the Roman Catholic Church became a powerful institution in Europe, the blessings of a priest became a necessary step for a marriage to be legally recognized. By the eighth century, marriage was widely accepted in the Catholic church as a sacrament, or a ceremony to bestow God's grace
I also have heard this from other sources, that marriage was a non religious institution in the begining.
But according to this,
Historians say the words were recited by a bride of Sumerian King Shu-Sin, fourth ruler of the Third Dynasty of Ur, who reigned between 2037 and 2029 BC, and used as a script for a ceremonial recreation of the sacred marriage.
According to the Sumerian belief, it was a sacred duty for the king to marry a priestess every year in order to make the soil and women fertile. The ritual of sacred marriage involved the re-enactment of the union of two deities, usually Inanna/Ishtar and Dumuzi/Tammuz. Thus, the priestess represented Inanna, the goddess of fertility and sexual love, while the king represented Dumuzi, on the eve of their union.
Here they talk about sacred ritual, beliefs, gods, and this happened between 2037 and 2029 BC. So there are only 300 years between 2029/2037 BC and 2350 BC, in a period of time where I doubt there were many scriptures refering to the nature of marriage, where marriage could have been a non religious thing.
Is there any written evidence stating marriage wasnt a religious institution in its origins?