In a New York Times column  defending traditional marriage, the fair-minded Ross Douthat argues that this venerable institution is “worthy of distinctive recognition and support” not because it is “natural,” but because when it works well it provides society “a microcosm of civilization, and an organic connection between human generations.” This conception of marriage, he adds, is unique to Western civilization on account of its Judeo-Christian roots.
Douthat denies that traditional marriage is “obviously natural in the way that most Americans understand the term.” In his view, “natural” today means “congruent with our biological instincts” (emphasis added). He then illustrates “in crudely Darwinian terms” how the male and female sexual instincts run counter to monogamous marriage.
The soundness of that argument aside, Douthat’s Darwinian understanding of “natural” exemplifies a point made by Cardinal Joseph Ratzinger that is critical to defending traditional marriage: the natural law, which was once the foundation of the Church’s dialogues with secular society “in order to appeal to the reason we share in common and to seek the basis for a consensus about the ethical principles of law in a secular, pluralistic society…has become blunt.” Why?
The idea of the natural law presupposed a concept of nature in which nature and reason overlap, since nature itself is rational. With the victory of the theory of evolution, this view of nature has capsized: nowadays, we think that nature as such is not rational, even if there is rational behavior in nature.
For Ratzinger, the same-sex marriage onslaught does not originate in the sexual revolution of the 1960s. It lies deeper in modernity’s rejection of the logos – the meaning of being – in the world, from which morality and the truth about human nature derive. Darwin’s theory of evolution, or at least some readings of it, is a definitive break from the logos of creation. It asserts that our behavior does not follow logically from a created order, which is the essence of the natural law, but is the result of instincts whose origins stem from mere chance. Nature and human instincts are products of random, irrational occurrences, and therefore no universal law, rule, or moral principle can originate from them.
Does this render eloquent explanations of traditional marriage based on the natural law, such as the one offered  by The Catholic Thing’s resident natural law expert Hadley Arkes, irrelevant? No, but Ratzinger is right: natural law reasoning sounds like a foreign language to those who have absorbed a Darwinian world view. Natural law depends on reason, and post modernity has lost faith in reason.
Many Americans simply reject the physical complementarity of the sexes as the basis for marriage. In their view marriage is based on the choice of two adults; procreation is reduced to a life-style choice. Thus instinct undirected by reason now drives the institution of marriage, and it propels the argument for broader sexual libertinism as well. The vulgar chorus of a song very popular among Millennials captures this sentiment well: “You and me baby ain’t nothin’ but mammals, so let’s do it like they do on the Discovery Channel.”
But Ratzinger’s belief that the natural law has become “blunt” does not necessarily mean “useless.” If the natural law is valid for all – and believers have the added assurance of revelation about its validity – then it can never be completely extinct. Reason, like many other realities, exists even if we refuse to believe in it.
Given the current difficulties, Ratzinger supplements the natural law understanding of marriage and morality on “personalist” grounds, that is, from the perspective of the human subject and his longing for fulfillment. Ratzinger’s defense of traditional marriage, articulated in God and the World, turns on his personalistic interpretation of the Sixth Commandment, “which not merely protects the future of mankind, but at the same time integrates human sexuality into human existence as a whole and, in that way alone, gives it its true value and stature for humanity.”
For Ratzinger procreation within marriage is more than a way to continue the species: the act itself “touches the holiness, the mystery of human existence, which goes beyond the realm of what I can control and dispose of. I simply do not belong to myself alone.” This approach may be the only effective reply to the argument that two consenting adults behind closed doors can do as they please.
The personalist perspective has a very important role today, but it alone is not enough. Personalism requires natural law to direct the subject’s longings, and natural law further requires classical metaphysics – the “logos of creation” – to be understood. But metaphysics has been besieged so steadily since the sixteenth century that, outside the Catholic Church, it is virtually unknown under the reigning dictatorship of relativism. Ratzinger calls modernity “the scientific age.” The future of metaphysics – and with it marriage in the West – may depend on Catholic scientists challenging the Darwinian world view, if not Darwin himself, on its own terms.
The struggle over marriage and its relation to the natural law is a crucial battle in a wider war over the very meaning of existence. As Benedict has often said, it matters a lot whether we think reason existed before the beginning of the world, or whether it arose somehow from blind forces. In an age where meaning derives from subjective sentiment, it is no wonder that the logic of the natural law seems an ironic victim of natural selection.