What would the triumph of same-sex marriage mean for American civilization? Americans disagree on this question. Liberals think of it merely as an incremental step toward justice understood as equality. For them, homosexuals have been unjustly excluded from marriage, and now they no longer will be. Nothing more momentous is involved.

Conservatives, on the other hand, think of same-sex marriage not as an extension of marriage but as a radical redefinition of it. To tamper with the very definition of a fundamental social institution like marriage, they warn, is to invite all manner of threatening consequences.

The conservatives are closer to the truth than the liberals on this question, but their foreboding does not go far enough. To embrace same-sex marriage is to plunge headlong into the abyss of nihilism. It is to step into a realm in which there are no longer any solid or reliable public standards of judgment as to what is right and wrong, just and unjust. It goes without saying, I hope, that this is not what the defenders of same-sex marriage intend. It is nevertheless the end toward which their position tends.

Tradition plays a larger role in some societies than in others. Put another way, some societies are more dynamic and forward-looking than others. Nevertheless, tradition is an important source of public standards in all societies. No community can afford to be so “progressive” as to disregard tradition entirely. To do so would be, in principle, to embrace chaos, since it would require constant renegotiation of the rules by which its members interact.

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There is, however, no older or more widespread notion than that marriage is a union between a man and a woman. It would be no exaggeration to call this definition a tradition of the human race. It is safe to say that a society that rejects this definition has also, whether it wants to admit it or not, rejected the idea that tradition should exert any authority over the present.

America, however, has never been guided only by tradition. It has looked to other and loftier sources of guidance. America is a branch of Western civilization, which has been defined since its origins by its quest for principles of judgment more solid than any particular society’s traditions, and by its willingness to judge its own traditions by these higher standards.

Perhaps, then, we could reject the traditional understanding of marriage without making the leap into nihilism, since we could still orient ourselves by the higher principles of right to which we have always been dedicated. A few moments’ reflection, however, would show that affirming same-sex marriage necessarily entails rejecting these higher principles as well.

As Leo Strauss famously observed, for Western civilization those higher standards have come from faith and reason, or from biblical revelation, on the one hand, and nature as understood by philosophy, on the other. Same-sex marriage is incompatible with both of these sources of wisdom.

The Bible could hardly be clearer that marriage is a union between a man and a woman. In both the Old and New Testaments it never speaks of marriage as anything else. When it speaks of sexual unions outside of marriage, it identifies them in order to disapprove of them.

No doubt many liberal Christians will want to deny such claims, but they really cannot do so without throwing overboard the very basis of their own identity as Christians. If the Bible does not affirm that marriage is a union between a man and a woman, then it does not affirm anything. If the Bible can be interpreted to affirm same-sex marriage, and if we are not to disclaim such an interpretation as pure sophistry, then it can be interpreted to mean whatever anyone happens to want it to mean.

Alternatively, we might admit that the Bible understands marriage as a union of a man and a woman and simply deny that we need be guided by it. In either case biblical revelation is not really operative as a source of authoritative moral wisdom. It is fair to say that a society that embraces same-sex marriage has chosen—again, whether it admits it or not—to turn its back on biblical revelation.

Of course, many liberal secularists would welcome this last abandonment of any public authority for revelation; but they would do well to ask whether our civilization could have become what it is, including all the things that liberal secularists approve (such as respect for equality and the dignity of the individual) without the moral influence of biblical revelation.

But one might also object that American civilization does not depend finally on biblical revelation as the source of its moral knowledge. While biblical religion has been important for many Americans, America has, in its authoritative public statements of its creed, looked not to revelation but to nature as understood by reason, its moral guide.

The Declaration of Independence, it might be said, looks not so much to the God of the Bible as to “the laws of nature and nature’s God,” which can be known by reason, and can be “self-evident” to the human mind. Perhaps, then, we can embrace same-sex marriage, thus throwing overboard our respect for the Bible, while still retaining a solid standard of public ethics in the natural-rights theory of the Declaration.

But same-sex marriage is just as incompatible with the thought informing the Declaration as it is with biblical morality. The Declaration’s arguments are part of a philosophic tradition that upheld reason’s ability to discern certain ends or purposes in nature and derive from them standards of human conduct. For this tradition, nature is morally intelligible.

Now, if normative principles and guiding purposes can be gleaned from our reflections on nature, there would seem to be no truth more obvious than that human sexuality is oriented by its nature toward uniting man and woman, with a view to procreation and family life. If nature cannot tell us that, then it cannot tell us anything. It is completely mute, and human beings are utterly left adrift to invent themselves according to whatever standards they like. This is nihilism, and this unfettered autonomy is in fact what the most consistent proponents of same-sex marriage admit they are after.

The rights doctrine of the Declaration—and the Lockean teaching from which it is derived—depend on reason’s ability to discern and draw valid conclusions from the data of nature. That all human beings have certain fundamental rights, says Locke, is evident from the fact that we are all the same kind of being, endowed with the same natural faculties. Among such beings, one cannot be made by nature to rule over another in the same way that we might rule a dog or a horse.

Yet it is surely no less evident that human beings are distinguished as male or female, that their sexual desires commonly lead these different kinds of human beings to unite, and that the common natural consequence of those unions is the procreation and nurturing of new human beings. It is incoherent to be guided by nature when it comes to individual rights but then to censor the data of nature when it comes to sexual ethics and the definition of marriage. To try it is to embrace a willfulness that is in principle indistinguishable from nihilism and that therefore cannot help but terminate in full-blown nihilism. There is, after all, no important difference between saying, “we only have to listen to nature when we want to” and saying, “we don’t have to listen to nature at all.”

We cannot embrace same-sex marriage and live in continuity with our past as a civilization. To embrace it is to deny that tradition, revelation, reason, and nature have any authority over us. We can make society whatever we want it to be. What will it be? No one can say for sure.

I do not expect that these reflections will mean much to many contemporary liberals. They seem exhilarated at the prospect of total liberation from the past. They think such liberation is itself a form of progress, and they are not ready to consider that it might in fact be a form of decay. Perhaps these arguments will, however, prompt second thoughts in those conservatives who regard the fight over same-sex marriage as a pointless distraction, who think conservatism should drop its opposition to the redefinition of marriage and get about the important business of defending free-market capitalism. Such conservatives should understand that the kind of audacity that can insist on redefining marriage is not likely to bow down with reverence before such concepts as property, contracts, and economic liberty.