Augustine on Marriage

in brief:

Forasmuch as each man is a part of the human race, and human nature is something social, and has in itself the power of friendship for a great and natural good; on this account God willed to create all men out of one, in order that they might be held in their society not only by likeness of kind, but also by bond of kindred. Therefore the first natural bond of human society is man and wife...

And this is the reason why marriage is not within the power of human civilizations to change--marriage is the foundation and fundament on which all societies rest, the basic fabric of human relations from which all other relations flow. As such, it is prior to society, and so society may do little other than choose between monogamy and polygamy, between no-fault divorce and no way to divorce. Marriage itself, like the earth under our feet, is a necessary precursor to our individual and societal existence.