Douglas Farrow, Professor of Christian Thought at McGill University (Montreal, Canada), writes:
The third-century Roman jurist, Modestinus, captured the common understanding of marriage with the following definition: “Marriage is the union of a man and a woman, a consortium for the whole of life involving the communication of divine and human rights.” This union and these rights exist, not merely for their own sake, but also and especially for the sake of the inter-generational concerns of progeny and property; with a view, that is, to the conditions necessary for the founding and flourishing of the family. The rights involved are divine as well as human because marriage is generative, and hence pre- as well as pro-political; because what is founded through marriage is, in the twentieth-century language of the Universal Declaration of Human Rights, “the natural and fundamental group unit of society.”
The same elements that found expression in Modestinus perdured and prospered in the Augustinian understanding of marriage as an institution entailing, not one, but three interwoven goods: proles, fides, et sacramentum—procreation or fruitfulness, loyalty or faithfulness, and bonding or sacred union. That societies shaped by this understanding took the unusual step of making marriage monogamous testifies to the seriousness with which each of these goods was regarded, precisely in its service to the others. It was by developing them in their mutuality, moreover, that heterosexual monogamy (to use the language of its detractors) created the conditions for the new and deeper respect for women and for children that until recently has characterized the West.
But marriage for some time has been under feminist attack for its putative institutionalization, in the name of divine rights, of oppressive patriarchal tendencies. This attack—coordinated, as it now is, with a Rawlsian assault on religious or comprehensive doctrines in the public sphere—has helped create a very different set of conditions, the conditions necessary for the advent of same-sex marriage. And same-sex marriage, by eliminating the first good (proles), has begun to unravel the whole fabric of marriage, setting up something else in its place: an institution not intrinsically connected to the family, or at all events not connected to the natural family. The divine and human rights belonging to marriage are thus beginning to disappear.
– “Why Fight Same-Sex Marriage? Is There Really That Much at Stake” (Touchstone)