In a brief filed with the Supreme Court last week, the Obama administration slammed the unusual legal argument now key in the movement against gay marriage: that gay couples cannot become accidentally pregnant and thus do not need access to marriage.

The argument has become the centerpiece of two major cases addressing gay marriage that the Supreme Court will consider at the end of March, Hollingsworth v. Perry, a challenge to California’s gay marriage ban, and United States v. Windsor, which seeks to overturn the federal Defense of Marriage Act.

"Only a man and a woman can beget a child together without advance planning, which means that opposite-sex couples have a unique tendency to produce unplanned and unintended offspring," wrote Paul Clement, a prominent attorney representing congressional Republicans in the DOMA case.

Clement added in his brief to the Supreme Court arguing to uphold that law that the government has a legitimate interest in solely recognizing marriages between men and women because it encourages them to form stable family units.

"Because same-sex relationships cannot naturally produce offspring, they do not implicate the State’s interest in responsible procreation and childrearing in the same way that opposite-sex relationships do," attorneys who are seeking to uphold Proposition 8, which banned gay marriage in California in 2008, argued in their brief. The opponents to gay marriage also argue it's possible the public perception of marriage would change if gay couples were allowed to wed, discouraging straight people from marrying.

In the administration's friend of the court brief, the Justice Department took a dim view of the argument.

"Marriage is far more than a societal means of dealing with unintended pregnancies," the Justice Department wrote. The brief also argued that preventing gay couples from marrying would not help or hurt the quest to encourage straight couples to marry when they have children.

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The argument for the government's right to ban gay marriage has evolved over the years. When the Supreme Court was first asked to address the issue in the 1970s—when a gay couple sued Minnesota for the right to legally wed—the justices replied that the request did not even raise a federal question worth answering. Those who wanted to prevent gay marriage argued that the federal government was not discriminating against anyone in adhering to a definition of marriage that had prevailed for centuries.

That was by and large enough of a legal argument to win the day every time, until the Massachusetts state Supreme Court became the first court to legalize same-sex marriage in 2003. The court ruled that the government had no legitimate reason to deny the recognition of marriage to its residents based on sexual orientation.

The one justice who dissented in the ruling, Robert Cordy, is credited with introducing the unintended pregnancy concept in his dissent, when he explained that the government does have a stake in defining marriage as only between men and women. Cordy argued that providing the benefit of legally recognized marriage coaxes straight couples into forming stable family relationships when they have children, which helps society as a whole.

An "orderly society requires some mechanism for coping with the fact that sexual intercourse [between a man and a woman] commonly results in pregnancy and childbirth. The institution of marriage is that mechanism," he wrote. The institution of marriage sends a message to men that they must help rear children, and thus the state has an interest in encouraging it so that fewer children are raised with only one parent. The state has no such obligation to encourage same-sex couples to wed, however, since they can only procreate together by making a decision to adopt or to use reproductive technology.

Since 2003, Cordy's reasoning has been cited in nearly every gay marriage case, and an evolved version of it is seen in the Proposition 8 case and the challenge to the DOMA law, which prevents the federal government from recognizing same-sex marriages. Gay marriage is recognized in nine states and the District of Columbia.

One of many potential pitfalls of the argument is whether it follows that the government could pass a law saying that only fertile people are allowed to wed, for example. Or whether the state could ban marriage between elderly people.

"I think there are going to be some justices who are extremely skeptical of it," said Doug NeJaime, a professor at Loyola Law School.