In November 2008, after a contentious political battle, the voters of California approved an amendment to their state constitution restoring their historic definition of marriage. “Only marriage between a man and a woman is valid and recognized in California,” read Proposition 8, California’s Marriage Protection Act.

A populist response to a California supreme court decision redefining marriage to include same-sex relationships, Prop 8 merely codified (like similar statutes adopted by nearly thirty states) what had always been the case in custom and practice in California—marriage is between a man and a woman.

The one-sentence amendment was meant simply to protect the definition of marriage from social (and judicial) revisionists. It was challenged in Perry v. Schwarzenegger, where plaintiffs Kristen Perry and Sandra Stier claimed they were denied a marriage license based on their sex and sexual orientation.

US District Court Judge Vaughn Walker agreed: “Perry is prohibited from marrying Stier, a woman, because Perry is a woman. If Perry were a man, Proposition 8 would not prohibit the marriage. Thus, Proposition 8 operates to restrict Perry’s choice of marital partner because of her sex.”

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Walker upended Prop 8 in August 2010, claiming that it “singl[es] out gay men and lesbians for denial of a marriage license,” thus preventing the state “from fulfilling its obligation to provide marriages on an equal basis.” He reasoned:

The evidence shows conclusively that Proposition 8 enacts, without reason, a private moral view that same-sex couples are inferior to opposite-sex couples . . . Because Proposition 8 disadvantages gays and lesbians without any rational justification, Proposition 8 violates the Equal Protection Clause of the Fourteenth Amendment.

At the heart of Perry, and the issue now before the Supreme Court, is whether or not marriage is discriminatory. Does Prop 8 unfairly discriminate against gays and lesbians, as Walker claimed, by denying them marriage licenses?

First, it’s worth asking what it means to discriminate. Is it always wrong to discriminate?

We have to admit that discrimination is part of daily life. Most of our decisions involve discriminating choices—from the food we eat, to the shops we patronize, to the movies we watch. A loving mother who forbids her children from watching certain television shows discriminates against those shows. A concerned father who refuses to let his daughter date certain shiftless young men discriminates against those men. Discrimination is a daily exercise that can serve us well if practiced well. In an Aristotelian sense, it is a virtue, the sharpening of which assists human flourishing.

So, when is discrimination wrong? Generally, when it is applied to persons who are unfairly treated as persons. Slavery, that great sin of our young republic, was morally wrong because it treated a class of persons as less than persons.

Race, nationality, and skin color are properties irrelevant to one’s personhood. There is nothing in skin tone that could restrict one from the rights and equality enjoyed by all citizens. That is why the Supreme Court struck down the “miscegenation” laws against interracial marriage in Loving v. Virginia.

According to Black’s Law Dictionary, unfair or invidious discrimination is the arbitrary granting of certain privileges to a particular class of persons within a larger group where “no reasonable distinctions” can be found between those favored and those unfavored. In other words, it means treating equals unequally.

There are at least two reasons to think that Prop 8 does not discriminate wrongly—that it does not treat equals unequally.

First, it does not target the sexual activity of gays and lesbians, nor does it purport to harm them. Prop 8, like other marriage amendments, is neutral on the question of whether the parties requesting a marriage license are heterosexual, homosexual, or asexual. A homosexual male may marry a heterosexual female. A lesbian may marry a straight male. The amendment only stipulates that the parties be of opposite sex.

As the Family Research Council noted in an amicus curiae brief to the Ninth Circuit Court of Appeals,

The fundamental flaw with the district court’s holding that Proposition 8 discriminates on the basis of sex is that “the marriage laws are facially neutral; they do not single out men or women as a class for disparate treatment, but rather prohibit men and women equally from marrying a person of the same sex.” . . . “[T]here is no discrete class subject to differential treatment solely on the basis of sex; each sex is equally prohibited from precisely the same conduct.” . . . Other courts have also rejected the claim that “defining marriage as the union of one man and one woman discriminates on the basis of sex.”

The “other courts” referred to above are many; the Institute for Marriage and Public Policy lists at least ten such federal and state high court decisions.

Second, to make a point that has perhaps been overlooked, same-sex marriage advocates (including Walker) presume that same-sex couplings are substantively equal to marital unions; they conflate discrimination against particular partnerships with discrimination against persons.

This is the fallacy of composition: One claims that what is true of the parts is also true of the whole. A basketball team may have five outstanding individual players, each capable of double-digit goals. But it would be a mistake to think this guarantees the team will have a winning season, unless they are coached well, learn to defend well, and work together as a fluid unit. In other words, the team is greater than the sum of its players.

Likewise, a marriage is greater than the sum of its two members. A same-sex coupling of two individuals cannot achieve the comprehensive union that makes a marriage—a union of hearts, minds, and bodies that is oriented toward sharing of family life—and is therefore something vastly different from, and hardly the equivalent of, a marital union. Thus, it’s false to say that Prop 8 codified unfair discrimination if marriage, by definition, cannot include same-sex relationships.

An individual who wishes to become a member of a club, society, or institution must meet the necessary conditions for entry. Boy Scouts are trustworthy, loyal, helpful, cheerful, and friendly, exhibiting these virtues to all persons equally and indiscriminately. Nevertheless, girls cannot join their institution because they are not boys. Of course neither can boys join the Girl Scouts.

The point is that while discrimination against gays and lesbians as persons is inappropriate, discrimination against same-sex couplings may be entirely appropriate if the coupling does not meet the necessary conditions of marriage. If reasonable distinctions are found between a same-sex coupling and a marital union, such that the coupling does not qualify to enter matrimony, then Prop 8 does not discriminate unfairly.

Of course, to make a coherent case that Prop 8 does not discriminate wrongly requires us to consider further the definition of marriage. Walker regards marriage as something malleable—something that has evolved over time such that “gender is no longer relevant” and “no longer forms an essential part of marriage.”

But if that’s true, then, as Sherif Girgis, Ryan Anderson, and Robert George have argued persuasively, we may rightly ask just what remains relevant in marriage if gender is no longer relevant. In this view marriage is a social convention; there is nothing intrinsically valuable in it beyond its construction by human beings at any point in history.

By contrast, in the historical view of marriage, it is a pre-political institution, an intrinsically valuable, basic human good rooted in human nature, not human will. As the Minnesota Supreme Court said in Baker v. Nelson, “[t]he institution of marriage as a union of man and woman, uniquely involving the procreation and rearing of children within a family, is as old as the book of Genesis.”

Indeed, Aristotle noted that the very foundation of the polis is the coming together of man and woman to create a family—the first fruits of society. Conversely, he was highly critical of homosexual conduct, believing it to be harmful to individuals and to the public good.

Among the late classical pagan writers, Plutarch perhaps expresses best the intrinsic goodness of marriage. In the Solon, he notes the public good of marriage and has high praise for laws that encourage love, honor, equality, and chastity between the spouses. And in his Erotikos (Dialogue on Love), he elevates the qualities of marital love: the heterosexual reciprocity, the equal status of the spouses, and the unitary aspects of the marital union.

It should not be lost that the ancients in no way regarded marriage as a social convention. They recognized marriage as the cornerstone of civilization. Greg Koukl states it well: “The truth is, it is not culture that constructs marriages or the families that marriages begin. Rather, it is the other way around: Marriage and family construct culture. As the building blocks of civilization, families are logically prior to society as the parts are to the whole.”

What is prior to society, then, cannot be changed, reinvented, or redefined by society. Walker may view same-sex unions as a social construct, but his view of marriage as a social convention is at odds with history.

Girgis, Anderson, and George have thoughtfully and thoroughly defended a sexual complementarity view of marriage; that is, the bodily joining of husband and wife, sealed in consummation and oriented toward children. They argue that marriage includes bodily, emotional, volitional, and intellectual shareholding unique to the institution.

Whether or not a marriage produces children, the inherent goodness of marriage remains whenever husband and wife become “one flesh,” coordinated toward the one biological ability they possess only incompletely without the opposite sex: reproduction. While gays and lesbians can form loving, monogamous, and emotionally supportive relationships, what they cannot achieve is sexual complementarity and its procreative potential.

As Girgis, Anderson, and George also point out, if gender is no longer relevant to marriage, by what normative principle can we exclude polygamous or polyamorous relationships? Suppose Kristin Perry and Sandra Stier were joined by Jane Smith? Could not three lesbians in a union be just as loving and committed to one another as a couple? By what principle can Perry exclude a threesome (or more) from obtaining a marriage license? At what point does the definition of “marriage” become so malleable that it becomes irrelevant altogether?

Whatever same-sex partnerships are, they cannot be called marriages because they do not meet the necessary conditions. They do not naturally form families, nor have they built civilizations or stabilized societies. A same-sex coupling is a social construction that shares little with the essence of marriage, and is therefore categorically distinct from a marital union. Where reasonable distinctions apply the charge of discrimination is unfounded.

The Supreme Court should recognize that Prop 8 does not discriminate against gays and lesbians, or any individual for that matter. It is facially neutral concerning sex and sexual orientation. Where it does discriminate, and properly so, is in preventing same-sex couples from obtaining marriage licenses, not because they are gays or lesbians, but because they cannot, as a couple, enter the institution of marriage as that term has always been defined.