Several years ago the trial judge presiding over the federal constitutional challenge to California’s Proposition 8 asked Charles J. Cooper, the lead lawyer defending the voter-approved measure, how the recognition of same-sex marriages affected heterosexual couples. Apparently caught by surprise, Cooper, a former assistant attorney general under President Ronald Reagan, candidly answered that he did not know.

Cooper will undoubtedly be better prepared to answer a version of the same question when he appears before the Supreme Court this week. The brief he filed with the court explains that the recognition of same-sex marriage disconnects marriage from procreation and that heterosexuals are less likely to procreate responsibly if gay couples are permitted to marry. The brief cites studies showing that nearly half of all pregnancies in the U.S. are unintended and that children do better when raised by married parents. According to the brief, the institution of marriage was created to address the biological reality that different-sex couples can procreate. In contrast, society does not have a similar interest in allowing same-sex couples to marry because their sexual intimacy does not lead to the creation of children.

There are several reasons why the Supreme Court should reject the effort to defend same-sex marriage bans based on how and when heterosexuals have babies. First, the historical record shows that marriage rates began dropping — and that cohabitation, divorce and out-of-wedlock birth rates began rising — long before Massachusetts in 2004 became the first state to recognize same-sex marriages.

The numbers tell the story. In 1970, unmarried cohabiting couples were raising 197,000 children under the age of 15. By 2000, that figure was up to 1,675,000, an astounding 750 percent increase. Furthermore, during those 30 years, the percentage of births that took place outside of marriage increased from 11 percent to 34 percent, while the number of single-parent households grew by 250 percent. At the same time, the marriage rate dropped by 22 percent and the divorce rate increased by 17 percent.

There are many reasons for these striking changes, including shifting social norms regarding the acceptability of cohabitation, divorce and single parenting. But the important point is that the changes would not have taken place had heterosexuals not decisively embraced greater personal choice and freedom in matters related to intimate and familial relationships. The push for same-sex marriage did not fray the link between marriage and procreation; heterosexuals did that all on their own.

The lack of a connection between same-sex relationships and how heterosexuals conduct their personal relationships is also evident in what has happened since some states began recognizing same-sex marriages. For example, although the rate at which Americans marry has been falling for decades, the drop in the national marriage rate since 2004 has been more than twice as large as the drop in the Massachusetts marriage rate. In contrast, the drop in the marriage rates in several of the states — such as Arkansas, Louisiana and Ohio — that in 2004 amended their constitutions to prohibit same-sex marriages has been several times greater than the national average.

It is also worth noting that the out-of-wedlock birth rates in same-sex-marriage states such as Connecticut, Iowa and Massachusetts are lower than the national average. In addition, in 2011, 4 out of the 10 states with the lowest divorce rates recognized same-sex marriages, with Iowa having the lowest divorce rate in the nation.

There is simply no evidence that allowing same-sex couples to marry affects heterosexual relationships. But let us suppose for a moment that the data showed that straight couples are more likely to have children within marriage if same-sex couples are prohibited from marrying. Even if this were true, it would still be morally problematic to deny same-sex couples the opportunity to marry based on that ground because doing so would disadvantage lesbians and gay men in order to advance the interests of heterosexuals. We do not have a tradition in this country of denying legal benefits to one group of individuals as a way of encouraging another group to act more responsibly. Penalizing same-sex couples in order to create appropriate procreative incentives for heterosexuals is simply unfair.

It is not just same-sex couples who are disadvantaged when they are denied access to the hundreds of rights and benefits that accompany marriage; their children are as well. It is improper for the government to harm some children — by denying them the social and legal support that comes with having married parents — in order to encourage some adults to have children within marriage. Even if the children of heterosexuals are better off when their parents marry, and even if heterosexuals are more likely to marry if lesbians and gay men are prohibited from doing so, that does not justify denying an entirely separate set of children the social and legal benefits that accompany marriage.

The legal defense of Proposition 8 is built around the claim that it is constitutionally permissible to prohibit same-sex couples from marrying in order to promote responsible procreation among heterosexuals. It turns out, however, that the claim is historically unsound, factually unsupportable and morally indefensible.

PHOTO: Mark Smith (L), and Todd Manoli stand at the alter during their marriage ceremony at Shotgun Ceremonies, a Vegas-style wedding chapel in Seattle, Washington December 9, 2012. REUTERS/Cliff Despeaux