In order to have a discussion about marriage, we must understand what marriage is. Not just what marriage is for devout Christians or Jews or Muslims or Buddhists. We must understand what marriage means to a society and culture that has the institution of marriage (and that is all of them.)
What is the public purpose of marriage, as distinct from private reasons or religious/sacramental reasons? Why did governments in different cultures in different periods of history view marriage as important? Why did the ancient Roman emperor Augustus lecture the order of Equestrians, speaking harshly of the fact that so many of the members of that order had never bothered to get married?
Children. It's about children. Every society--- whether underpopulated or overpopulated--- needs a steady supply of children to be born and to be raised up into useful citizens. Marriage attaches mothers and fathers to their children, and to one another.
Compare children from a stable married home to children raised in state orphanages, as in Russia; to children of single mothers; to children in step-parent homes; to children in foster care. The children from a married-parent home win. And this is why marriage is an important public institution and why good governments act to preserve and promote it.
Note: "The essential public purpose of marriage is to attach mothers and fathers to the children and to one another" is reason #1 in '77 Non-Religious Reasons to Support Man/Woman Marriage'.