For better or for worse, same-sex marriage is one of the most successful social movements in American history. Its claims were outside the realm of political possibility as recently as the early 1990s. Now its victory is probably inevitable. It has succeeded largely because so many of its opponents have been so inarticulate, and—this is crucial—have failed to pass on their views to their children. According to Gallup, 46 percent of Americans oppose same-sex marriage, with 53 percent in favor. The percentage in support has doubled in only fifteen years. There is a sharp generational divide: among those eighteen to twenty-nine-years-old, 73 percent support same-sex marriages. That number drops steadily with age, to 39 percent of those 65 and older. The result has been a massive political shift. Barack Obama is the first Democratic president to support same-sex marriage. He is also the last Democratic president to oppose it. The Republicans have begun, painfully and grudgingly, to do likewise.

So What Is Marriage? is an important book. It is clear, tightly reasoned, and a remarkably fast read for a dense philosophical argument. It should be instantly recognized as the leading statement of the case against same-sex marriage, together with Maggie Gallagher’s half of Debating Same-Sex Marriage (coauthored with John Corvino). Gallagher’s strategy is consequentialist, turning on baleful but improbable predictions about the effect of same-sex marriage on heterosexual familes. The authors of What Is Marriage?, on the other hand—Sherif Girgis and Ryan Anderson are unusually bright graduate students, and Robert P. George is the McCormick Professor of Politics at Princeton—are proponents of the New Natural Law theory, a philosophical school whose leaders are the Catholic scholars Germain Grisez and John Finnis. They make some of Gallagher’s claims, but their central thesis is not a guess about consequences. Their theory’s central idea is that there are universal human goods. Its pertinent claim here is that marriage is such a good, “a distinctive kind of bond that has its own value and structure, which the state did not invent and has no power to redefine.” Its goodness arises from the bodily union that only a man and a woman can achieve.

The challenge, for opponents of same-sex marriage, has always been to explain what intrinsic difference there could be between same-sex and opposite-sex couples. The New Natural Lawyers have responded by claiming that heterosexual marriage has an intelligible essence, one in which same-sex couples cannot possibly participate.

Their characterization of that essence has shifted over time. Grisez wrote in 1993: “each animal is incomplete, for a male or a only a potential part of the mated pair, which is the complete organism...capable of reproducing sexually. This is true also of men and women: as mates who engage in sexual intercourse suited to initiate new life, they complete each other and become an organic unit. In doing so, it is literally true that “they become one flesh” (Gn 2.24)”. What looks like a metaphor in Genesis becomes here a statement of fact. The authors of What Is Marriage?, in an earlier paper, made the same claim: the married couple, when mating, “truly become biologically one, one body.” If this were true, it would indeed point to something in which same-sex couples cannot possibly participate. Other sexual acts, whether homosexual or heterosexual, cannot achieve this bodily unity. At best, they achieve the illusory experience of unity.

The trouble however is obvious: even when a couple conceives a child, they do not become a single organism. In reproduction, two entities share in a bodily action. That does not mean that they become one, even though the action they perform could not be performed by either of them individually. Two pianists playing a four hands piece do not become biologically one, even though they are using parts of their bodies in a complementary way.

In What Is Marriage?, this claim is silently abandoned in favor of a more modest one. Man and woman, in coitus, “coordinate toward a common biological end of the whole that they form together.” The consequence is a distinctive human good:

Just as one’s organs form a unity by coordinating for the biological good of the whole (one’s survival), so the bodies of a man and woman form a unity by coordination (coitus) for a biological good (reproduction) of their union as a whole. In choosing such biological coordination, spouses unite bodily, in a way that has generative significance.

A same-sex couple cannot achieve the same good, because “there is no bodily good or function toward which their bodies can coordinate.” In order for organic bodily union to occur, the two bodies’ organs must be coordinated toward something, and in human bodies, there is only one biological end that two persons’ organs can coordinate toward. (A four hands piano piece isn’t a biological union. Is a chorus, which coordinates voices toward what is arguably at least in part a bodily good, a harder case?) Because a same-sex couple cannot achieve this kind of bodily union, it is impossible for them to marry.

A central objection to this claim, one that I have pressed upon these authors elsewhere, is that the argument cannot explain why heterosexual couples who know themselves to be infertile are within the charmed circle: a sterile person’s genitals “are no more suitable for generation than an unloaded gun is suitable for shooting.” (Their characterizations of my objections are scrupulously fair and accurate.) When the couple is infertile, they reply, their bodies “are still united in coitus as much as organs of a single body are united: toward a single biological good (reproduction) of the whole that they compose together.”

This claim, unlike the single-organism notion, is coherent. A broken gun (even an irreparably broken gun) is still a gun, and its parts are still united with one another, oriented toward a purpose, even though they do not work properly. The same is not true of a pile of gun parts. The infertile heterosexual couple is united with one another in the same way in which the parts of a broken gun are united with one another.

But this move still leaves a puzzle about why the infertile heterosexual couple has achieved a good that the same-sex couple cannot achieve. They argue that the infertile couple’s union is “a valuable part of a valuable whole.” But what value would there be in deliberately assembling an irreparably broken gun? The product would have a kind of unity, but the goodness of that unity, as a reason for action, is mysterious. Is not the asserted intrinsic goodness of the procreative-type acts of infertile heterosexuals similar?

They also curiously fail to appreciate certain kinds of reproductive coordination. They think bodily union does not happen in artificial reproduction: “gametes that have been extracted and manipulated for laboratory use are hardly parts of the parents’ persons, so combining them could not possibly make for a bodily (hence personal) union of the parents.” But many marine invertebrates, such as sponges, reproduce sexually through “broadcast spawning” in which they release gametes into the ocean, where they fertilize externally. In those cases, are not the male and female coordinating together toward a bodily good?

Other aspects of their account of marriage are similarly mysterious. They say their account alone makes sense of widely held intuitions about marriage. The physical union of male and female, in their view, is appropriately part of a more comprehensive union. “Being organically united—as ‘one flesh’—spouses should have, by commitment, the exclusive and lifelong unity that the parts of a healthy organic body have by nature.” The “should” is a non sequitur. Monogamy is swell, but it doesn’t follow from biological unity; one person can coordinate bodily with multiple others. (Again, think of a chorus.) The authors claim that, unless their understanding of marriage is widely shared, social pressures will diminish “for husbands to stay with their wives and children, or for men and women to marry before having children.” This is because only that understanding can undergird marital stability: “As more people absorb the new law’s lesson that marriage is fundamentally about emotions, marriages will increasingly take on emotion’s tyrannical inconstancy.” But in fact, among the top economic quartile of Americans, who are most likely to endorse same-sex marriage, rates of nonmarital birth and divorce haven’t significantly changed since the 1950s. These people evidently perceive a reason to control emotion’s tyrannical inconstancy, a reason that eludes the authors of What Is Marriage? The authors’ understanding of marriage is so novel and esoteric that it is hard to believe that it has any effect at all on ordinary people’s behavior. They cite a few ancient philosophers whose conclusions about marriage are broadly consistent with theirs, but philosophy is about arguments, not conclusions.

They think that anyone who endorses same-sex marriage must believe that marriage is “essentially an emotional union, merely enhanced by whatever sexual activity the partners find agreeable.” Such a view can’t offer any principled boundaries to marriage, and so must logically sweep within it both polygamous groups and celibates who happen to share a household. They are right about the weaknesses of any rival essentialism. The most attractive alternative to their view is that marriage is not “essentially” anything. It is a contingent cultural formation, which doubtless would never have arisen if humans did not reproduce sexually, but which nonetheless has no essence. There are regularities about it that ought to influence how married people should behave. It’s handy to know that 99 percent of heterosexual couples expect sexual exclusivity, and that violations of it are the leading cause of divorce across dozens of different cultures. Marriage nonetheless might be a practice that suits human needs but which can be modified freely as our understanding of human needs changes.

Their case against same-sex marriage relies on other claims. Legal recognition of same-sex marriage would make it harder for people to realize the good of marriage, by obscuring their understanding of what that good is. They fear that widespread acceptance of same-sex marriage will lead their own views to be regarded as bigoted. As they acknowledge, these claims are dependent on the core thesis about the essence of marriage.

That claim’s most fundamental difficulty is the short distance from premise to conclusion. The union of the married heterosexual couple is uniquely good because...well, because the union of the married heterosexual couple is uniquely good. This raw intuition comes decorated with a complex theoretical apparatus, but that apparatus does no work. It’s like one of those old trick math problems, which at first glance seems to require complex computations:

7 + 8,398.14 × B ÷ √55 - 8,398.14 × √55 ÷ B = ?

Look again, and it’s clear that all the complexity cancels itself out, and that you end up right back where you began.

The publication of What Is Marriage? is a public service. It advances understanding of a perspective that many (though fewer and fewer) Americans share, but it is unlikely to persuade anyone who doesn’t already agree with its claims. It is a lucid window into a disappearing worldview.

Published in the 2013-05-03 issue: 
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