In the 1890s, the Chinese intellectual Kang Youwei attempted to modernise China along Western — that is, liberal — lines. He proclaimed the equality of humanity as well as a notion of individual autonomy. The Oneida experiment ended, however, when a generation of children was born and the parents lobbied to be allowed to marry and form stable family units. The parents chose attachment over radical autonomy.
Human nature is not putty in the hands of progressives. The earliest humans were monogamous, and there is no record of any society before the family. The family is a natural society, not a “social construct.” In Aristotle’s definition, it is ‘a society established by nature for the supply of men's everyday wants.’ People in a stable union aiming at a common end through common action form a society, and the primary end of the family is the good of the child — its existence and rearing.
Sex is the strongest of the passions and primarily directed toward the child. And raising a human child takes about twenty years — a duty that falls on both parents equally because they are equally the cause of the child’s existence. This requires a stable and exclusive union. Although the mother’s role is more important at the start, the father’s role becomes increasingly important with time: boys need both a mother’s breast and a father’s boundaries.
Marriage thus creates a contract uniting a man and a woman in a permanent and exclusive union primarily for the begetting and rearing of children. Nothing in human life can wreak havoc like sex can. A fling merely for the sake of procreation is a betrayal of the child. All sex outside marriage is wrong for the same reason masturbation and homosexuality are: attempting to get sexual satisfaction while avoiding the responsibility attached to it.