After one year as president of the Catholic University of America in Washington D.C., John Garvey took to the pages of the Wall Street Journal to announce a change in his university’s policy for housing students on campus: a return to all-male and all-female residence halls, and the gradual elimination of mixed-sex buildings. According to the Washington Post, Catholic University first changed to “co-ed” housing over two decades ago and currently houses both sexes in eleven of its seventeen residence halls—though men and women remain in separate floors or wings, unlike the latest fashion of shared suites, bathrooms, and even sleeping quarters at some universities.

President Garvey’s stated reason for separating the sexes into their own buildings, starting with the incoming freshmen in the fall of 2011, is to combat the pattern of binge drinking and “hooking up” among the students, and the consequent risks to body, mind, and soul of these behavior patterns. He made no claim that separate living arrangements would magically cure the ills he diagnosed. But why contribute to the problem when you can at least foster solutions?

It ought to be surprising that Catholic University ever experimented with co-ed housing. But this essay will not be about the University’s decision to reverse course on student residential policy.  It will instead be about a revealing remark made by one of the opponents of the decision.

Elsewhere in the District of Columbia, at George Washington University, law professor John Banzhaf announced that he intended to sue Catholic University for sex discrimination under the District’s Human Rights Act. Banzhaf, a formidable nuisance as a litigator, told Inside Higher Ed that separating the sexes was like a return to the old evil of “separate but equal” in racial segregation:

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“Suppose a university decided that there would be less racial tension if all the blacks were in a black dorm, all the whites were in a white dorm,” Banzhaf said. “Each one is, quote, getting their own dormitory, and maybe some of them would be happier that way. But surely no one would suggest that it’s lawful.” The statute does not require that a certain population be disadvantaged for an action to be illegal; the simple act of segregating the genders is enough, Banzhaf said.

Banzhaf may have a case under the D.C. Human Rights Act, or he may not. That will be for others to decide. But this parallel of his, between race and sex, is what should catch our attention. His argument, as a matter of justice and moral right, is only as good as the proposition that sex is just like race when it comes to our treatment of others. Banzhaf is sure that if it would be wrong to separate the races into different dormitories, even into facilities of identical quality, it would be equally wrong to separate the sexes.

But is sex just like race? Let us take it as given that justice demands a legal order that is, as the first Justice Harlan put it 115 years ago in Plessy v. Ferguson, “color-blind,” taking no notice of anyone’s race when it comes to his status or treatment in law and public policy. Would we say, in the same way, that the law should be “sex-blind,” taking no notice of the fact that some persons are men and others are women?

Even the Supreme Court has not gone this far. In perhaps its furthest-reaching sexual equality decision, United States v. Virginia (forcing Virginia Military Institute to admit female students in 1996), the Court still maintained that an “exceedingly persuasive justification” for treating the sexes differently would pass constitutional muster, in circumstances where racial distinctions would not. Considerations of privacy, safety, decency, and the virtue of members of both sexes would seem to be sufficient justification for separating the living accommodations of young men and women in college. The possibility that a policy of separation might run counter to the desires of many college-age men and women might only prove the justice of it.

And none of these considerations, turning on the risks and probabilities of sexual activity, with all the spin-off concerns about alcohol consumption, sexual assault, pregnancies and possible abortions, disease transmission, and plain old-fashioned “relationship problems,” would even merit our attention if we were asking whether the races should be separated. The best Professor Banzhaf can do is imagine that “racial tension” might be employed as a ground for separating the races. Our problem, when it comes to mixing or separating the sexes, is a bit more complicated than that, and begins with something rather the opposite of “tension.”

Banzhaf’s blithe parallel, however, of treating sex just like race is lately a favorite rhetorical turn of the campaign for same-sex marriage. The Supreme Court ruling cited by those making this argument is Loving v. Virginia (1967), in which the justices unanimously struck down the law of that state (and, by implication, those then remaining on the books in fifteen other states) against “miscegenation,” or inter-racial marriage.

The state of Virginia attempted to defend its policy by arguing, among other points, that the law treated both races equally, since it forbade whites to marry blacks, and blacks to marry whites, and assessed penalties under the law without regard to the race of those convicted. One hears similar defenses of marriage as a conjugal union between a man and a woman when the argument is made that the law treats heterosexual and homosexual alike, securing identical marriage rights to persons of either sexual orientation—to marry someone of the opposite sex. Is this formal-equality argument valid in both cases, or neither, or in one but not the other?

Chief Justice Earl Warren, writing for the Court in the Loving case, saw through the state’s claim to treat the races with formal equality when he noted that because the law prohibited “only interracial marriages involving white persons”—not, for instance, a marriage between a person of African descent and one of Asian descent—it was evident that the law was “designed to maintain White Supremacy.” In like vein, the advocates of same-sex marriage treat the argument that “gays and lesbians too can marry persons of the opposite sex” as a cruel joke, and focus on the fact that they are not permitted to marry the persons they wish to marry. Thus, their complaint runs, there seems to be some “Heterosexual Supremacy” at work in the determination to preserve marriage as it has always existed, as a union of man and woman.

As Chief Justice Warren pointed out, the anti-miscegenation laws of Virginia and other states “proscribe[d] generally accepted conduct if engaged in by members of different races,” and thus interfered with a “freedom of choice to marry” on grounds that were simply irrelevant to the marital relation: race, and race alone. If not for the laws against inter-racial marriage, a man and a woman of different races would have enjoyed the freedom to marry in the ordinary course of things, barring other difficulties that are relevant to the marital relation (age of consent, consanguinity, and the freedom of a single person who would not be committing bigamy by entering a new marriage).

But Warren’s reasoning makes no sense except on the tacit presumption of another consideration, so integral to the marital relation that in 1967 it did not occur to him to state it, although it is implicit in his phrase “generally accepted conduct”—and that is, that the two persons free and capable of marrying are a man and a woman. Virginia and other states, in their anti-miscegenation laws, had interfered with a natural relationship by introducing something—race—that was at right angles to it. To rule out bigamy, or to regulate the age of consent, or the degrees of permitted consanguinity—all these place conditions on the freedom to marry that are oriented toward the fulfillment of marriage’s purposes. But introducing race into those conditions injects another purpose—racial “purity”—into the institution, forcing it to serve an end alien to itself. When unmarried, unrelated adult men and women choose to marry, they enter into a relation that is good in itself, a relation of opposite-sex individuals capable of the kind of union, and the only kind of union, that naturally produces offspring. The laws of marriage can and should facilitate, regulate, and solemnize this relation, and provide that it endure. But when political authorities permitted some marriages, and prohibited others, on racial grounds, they were interfering with it in pursuit of goals foreign to its nature, and acting unjustly in two ways, both in their racial discrimination and in changing the meaning of marriage. They were, in short, instrumentalizing marriage as a tool of state policy rather than honoring its nature and fostering it as what it is.

The advocates of same-sex marriage attempt a Loving-style “freedom of choice to marry” argument when they challenge the laws now in place to restrict marriage to opposite-sex couples. In its 2009 ruling in Varnum v. Brien, for instance, the Iowa Supreme Court noted that in 1998 the state legislature had acted for the first time to “define marriage as a union between only a man and a woman.” The court went on to say that “except for [this] statutory restriction,” the same-sex couples applying for marriage licenses would have “met the legal requirements to marry in Iowa.”

But such a statement is flagrantly facetious. The Iowa legislature did not introduce a new, orthogonal consideration into the law of marriage in 1998. It did not, in fact, make new law at all, but only codified an age-old understanding, consistent with the nature of marriage itself, precisely in order to ward off a bizarre new challenge hitherto unheard of—the applications, and litigious responses to denied applications, of same-sex couples seeking to marry. Only the determined campaign of those who would redefine marriage had prompted Iowa legislators—and by now, the voters in 30 states who have amended their constitutions—to say anything at all in the law about the sex of those who are permitted to marry. In the normal course of things up until the last two decades, the law did not bother to state a “restriction” that was understood to be no restriction at all but merely a natural fact: men and women get married, but men and men or women and women do not.

Today it is those claiming a specious “freedom to marry” who make a claim at odds with the institution’s nature and alien to its purposes. It is they who would instrumentalize it by a redefinition, a destroying and remaking, that puts marriage to a new kind of work in the service of state policy. For race and sex are not, in the final analysis, really just like one another at all.  Race is an interesting cluster of facts about ancestry, history, culture, and geography; we can let it get in the way of human relations by making too much of it, or thinking about it in wrongheaded ways. We can, and for many purposes should, let it alone entirely.

Sex, on the other hand, is fundamental to our relations with one another. We cannot let it alone.  We cannot wish away its normal fruits of attraction, passion, and the generation of offspring, or its intimate connection with virtue and vice, wisdom and folly. Catholic University’s President Garvey is right to treat the relations of the sexes in the college population as a matter of the utmost moral gravity, justifying measures that would be intolerable if race were the difference under consideration. So too, in the debate over marriage, we must not be led astray by farcical assimilations of sex to race, as though differential treatment along those two dimensions were exactly alike, when they are not.

Matthew J. Franck is Director of the William E. and Carol G. Simon Center on Religion and the Constitution at the Witherspoon Institute.