Today’s Supreme Court oral argument in the same-sex marriage case touched on all of the different rationales for a constitutional right to same-sex marriage. It is hard to say which one the justices will ultimately settle on, or even if there will be a single rationale that gets the support of a five-justice majority.

One that I have a particular interest is the sex discrimination argument that was the focus of an amicus brief I coauthored with Northwestern law professor Andrew Koppelman. That argument in fact came up in an important question posed by Chief Justice Roberts to John Bursch, counsel for the states defending their laws banning same-sex marriage (kudos to legal scholar Eric Segall for correctly predicting that the Chief Justice would be the one to raise this issue):

Counsel, I’m not sure it’s necessary to get into sexual orientation to resolve the case. I mean, if Sue loves Joe and Tom loves Joe, Sue can marry him and Tom can’t. And the difference is based upon their different sex. Why isn’t that a straightforward question of sexual discrimination?

This is exactly the point that advocates of the sex discrimination theory have been making for years: Laws restricting marriage to opposite-sex couples allocate legal rights based on gender. That is the very essence of sex discrimination.

Bursch gave two answers to the question. He first claimed that the Court’s sex discrimination precedents don’t cover this case because all of them “involved [laws] treating classes of men and women differently,” whereas laws banning same-sex marriage restrict both men and women in the same way. Neither is permitted to marry another person of the same sex.

The problem with this argument is that, by the same reasoning, laws banning interracial marriage don’t discriminate on the basis of race. To paraphrase Busch, those laws did not, on their face, “treat classes of blacks and whites differently.” People of both races were equally forbidden to marry a member of the other. As we discuss in our amicus brief, the Court rejected this theory in its 1960s decisions striking down laws banning interracial sex and interracial marriage; the same sorts of considerations should lead it to reject it in the case of marriage laws that allocate legal rights based on gender. In addition to discriminating on their face, laws banning same-sex marriage are also in part based on sweeping (and often empirically dubious) generalizations about the abilities and social roles of men and women, much as laws banning interracial marriage were in large part motivated by racial prejudice (a point also discussed in detail in our brief). The Supreme Court has repeatedly ruled that laws that discriminate on the basis of sex cannot be justified by “fixed notions concerning the roles and abilities of males and females.”

Bursch’s second argument is more interesting. He suggested that laws banning same-sex marriage can be justified based on the Supreme Court’s decision in Tuan Anh Nguyen v. INS (2001), which upheld a federal law making it more difficult for foreign children of American fathers to acquire citizenship based on that relationship than for foreign children of American mothers to do so. If Nguyen is indeed the relevant precedent, that means that the same heightened judicial scrutiny that applies to other forms of sex discrimination also applies to laws banning same-sex marriage. The Nguyen Court applied heightened intermediate scrutiny to the legislation at issue in that case, which means that the law in question could only be upheld if it is substantially related to an important state interest. In Nguyen, the relevant interests were ensuring that the supposed father really was the child’s parent (which is more difficult in the case of fathers than mothers), and that he had a meaningful parental relationship to the child. By contrast, it is hard to find an important state interest that can only be achieved by excluding same-sex couples from marriage. The interest in child-raising, for example, is clearly served by same-sex couples as well, since they too raise children. Marriage is not and has never been limited to couples who can procreate by biological means. And in any event, as several justices noted, using marriage to promote the state’s interest in biological procreation does not require the exclusion of same-sex couples from the institution.

In a prolonged discussion of Bursch’s answer, several justices expressed skepticism about his reliance on Nguyen, including Ruth Bader Ginsburg, Elena Kagan, Sonia Sotomayor, and key swing voter Justice Anthony Kennedy (who was the author of the Court’s opinion in Nguyen, and raised the sex discrimination issue the last time same-sex marriage was before the Court, in the 2013 California Proposition 8 case). Justice Ginsburg offered the strongest response, noting that “in the Nguyen case, the father could get the status of a parent. He just had to do some things that the mother didn’t have to do. It wasn’t difficult. Here it’s a total exclusion.” If the law at issue in Nguyen had made it completely impossible for foreign-born children of American fathers to get citizenship, while automatically granting it to comparable foreign children of American mothers, the Supreme Court would almost certainly have invalidated it.

None of this necessarily shows that the Court will base its decision on the sex discrimination argument, or even that any individual justice will endorse it. If, as most people expect, the Court ends up striking down laws banning same-sex marriage, the justices have several possible paths to that outcome. But it does show that multiple justices are at least taking the sex discrimination issue seriously.

The justices’ interest in this issue may not have any connection to our brief. Perhaps they have focused on the theory in part because a number of recent lower court opinions relied on it. But whatever the reason, the sex discrimination argument is clearly in play.

UPDATE: It may be worth noting that Roberts’ formulation of the sex discrimination argument is very similar to ours, from page 8 of our brief: “Put simply, if Ann is permitted to marry Bob, but Charles may not marry Bob, then Charles is being discriminated against on the basis of sex because he is being treated “in a manner which but for [his] sex would be different.” Andrew Koppelman and I used similar phrasing in a recent USA Today op ed: “If same-sex marriage is forbidden, Anne is allowed to marry Bob, but Charles can’t. Charles is denied the right to marry Bob, solely because Charles is a man. Denial of a legal right solely because of gender is the very essence of sex discrimination.” That still doesn’t necessarily mean that the Chief Justice’s question was based on our brief. He certainly could have found a similar formulation elsewhere, or come up with it on his own.