Marriage by God?

Some consider marriage as made by God. But not all do. Marriage came out of early cultures. I hear it is stated that God, usually it is mentioned or implied the Christian God, created marriage. But, most or all of these early cultures had gods or a god while having an institution of marriage.
Unless one wants to argue that all these deities were true then cultures without this true deity created marriage. So, is marriage then a product of mankind or God?
Now, these cultures believed that their deity or deities endorsed marriage, but the West has moved beyond the idea that the government, the culture, the law should be a reflection of whatever religious text states. We now understand that the law is to be made for man, not for a deity which might or might not exist.
If one wants to follow a religious based law in all areas of one's life, fine, that is their right. If a church wants to restrict marriage to only heterosexual couples, fine. That again is their right. But, not all Americans are religious.
Marriage has changed tremendously in the past century. Blacks and whites couldn't marry in much of the country, less then 40 years ago. At one time marriages were arranged with the young husband and wife having no say in the matter. At one time women were denied basic rights in a marriage. She was a second-class citizen in her own marriage.
We have progressed and done away with these injustices. Gay Americans might or might not be religious, just as their fellow citizens might or might not be. but they deserve the same rights under secular marriage laws as all others to marry according to their natural sexual orientation. It is as unjust to deny that right as it would be to deny a man or woman to marry the opposite sex while limiting them to only the same sex.
Gay men and women pay into the system, be it Social Security taxes, income taxes, property taxes, and should not be treated by that same society as second-class citizens.