Not long into the Supreme Court arguments Tuesday in Hollingsworth v. Perry, Justice Elena Kagan put her finger on the implausibility of the central constitutional argument made by the lawyers defending California’s Proposition 8, which defines marriage as between a man and a woman. In reading the briefs, she said, she was struck that the “principal argument” of gay-marriage opponents is that “the State’s principal interest in marriage is in regulating procreation.” She then offered a hypothetical. “Suppose a State said that, ‘Because we think that the focus of marriage really should be on procreation, we are not going to give marriage licenses anymore to any couple where both people are over the age of 55.’ Would that be constitutional?”

Here was the response by the lawyer for Proposition 8 supporters, Charles Cooper:

"[S]ociety's interest in responsible procreation isn't just with respect to the procreative capacities of the couple itself. The marital norm, which imposes the obligations of fidelity and monogamy, Your Honor, advances the interests in responsible procreation by making it more likely that neither party, including the fertile party to that … marriage will engage in irresponsible procreative conduct outside of that marriage. Outside of that marriage. That's the marital—that's the marital norm."

In other words, if the Court does decide the Perry case on the merits, it will come down to this claim: Because only straight people can impulsively and accidently have illegitimate children out of wedlock, they need a stable institution of marriage to discourage them from doing so and to force them to focus on the consequences of their animalistic passions. But as Justice Kagan noted, the idea that denying marriage equality to gay couples would encourage monogamy and responsible procreation by straight couples is hard to follow, let alone to fathom.

It’s also hard to imagine that more than a handful of the people who voted for Prop 8 in California were moved by this “responsible procreation” argument, which first surfaced in the 1990s in law review articles and presentations by Lynn Wardle, a law professor at Brigham Young University, and Teresa Collett, a law professor at the University of St. Thomas. The real reasons that motivated a majority of California voters to oppose gay marriage were moral disapproval of homosexuality (which is sometimes religiously based) and a desire to maintain the traditional definition of marriage, because of a sense that the institution is embattled and deserves as much stability as possible. For better or for worse, however, the Supreme Court has ruled that both of these reasons are constitutionally illegitimate. In the 1996 Virginia Military Institute case, Justice Ginsburg held that maintaining tradition for its own sake is not a constitutionally permissible goal. And in striking down sodomy laws in the 2003 Lawrence v. Texas case, Justice Anthony Kennedy held that moral disapproval is not a legitimate justification for a law either. “The fact a State’s governing majority has traditionally viewed a particular practice as immoral is not a sufficient reason for upholding a law prohibiting the practice,” he wrote. That’s why Justice Scalia was correct to predict in his Lawrence dissent that “this effectively decrees the end of all morals legislation,” and would lead inevitably to the legal recognition of same sex marriage.

Because the real reasons that motivated California voters to oppose gay marriage have been ruled out of bounds by the Court, Proposition 8 supporters, during the trial and in their Supreme Court briefs as well as during the oral arguments, have been forced to rest their entire case on the state’s interest in promoting “responsible procreation.” But the argument is so esoteric that it’s hard to state succinctly in an English sentence, let alone a paragraph or two. Here’s an attempt in the brief filed by 19 states that define marriage as between a man and a woman, led by Virginia Attorney General Ken Cuccinelli: