SAN FRANCISCO (AP) – For judges deciding whether same-sex couples can marry, much of the legal analysis boils down to sex. Or to put a finer point on it, procreation.

Opponents of same-sex marriage, such as the Arizona-based Christian legal group Alliance Defense Fund, argue that the need to ensure the survival of the state and species is the historical and institutional foundation upon which marriage exists.

“The fundamental right to marry has always been about procreation,” Alliance Defense Fund attorney Glen Lavy recently argued to San Francisco County Superior Court Judge Richard Kramer, who is expected to rule any day on California’s same-sex marriage ban.

Kramer is the latest among dozens of judges and courts nationwide to hear the argument that procreation is a loftier goal than ending what many same-sex couples say amounts to discrimination.

But before judges decide whether supporting procreation should outweigh ending alleged discrimination, they must first answer the question of whether there is a “rational basis” to limit the rights and privileges of one group of citizens – same-sex couples – to promote the legitimate state interest of fostering the human race’s survival.

Courts in Massachusetts, New York and Washington State recently wrote that such a state interest no longer applies in a world of artificial insemination and adoption. Those courts also said procreation isn’t threatened by same-sex marriage.

“The precise question is whether barring committed same-sex couples from the benefits of the civil marriage laws somehow serves the interest of encouraging procreation. There is no logical way in which it does so,” King County Superior Court Judge William Downing of Seattle ruled in August 2004.

But courts in Indiana and Arizona, facing the same sets of facts, have gone the other way.

The Indiana Court of Appeals ruled in January that “there was a rational basis for the Legislature to draw the line between opposite-sex couples, who as a generic group are biologically capable of reproducing, and same-sex couples, who are not.”

Edward Lazarus, a former Supreme Court clerk, said, “The states and opponents are trying to find criteria why marriages could only be reserved for men and women.”

Voters, of course, are beholden to no legal balancing tests as they go to the polls.

Voters overwhelmingly passed constitutional amendments banning same-sex marriage in all 11 states where the question appeared on the November ballot, perhaps motivated by San Francisco’s brazen move to sanction 4,000 same-sex weddings and the ruling by the Massachusetts Supreme Judicial Court to legalize same-sex marriage in that state. The California Supreme Court eventually nullified the San Francisco unions.

Unlike voters, judges must find legal precedent for their decisions.

When it comes to same-sex marriage, previous precedents are being challenged like never before. Same-sex couples only recently have begun forcing the issue in court, experimenting with lawsuits that were rarely broached in past decades when courts barred mixed-raced marriages and allowed schools to fire gay and lesbian teachers.

Added to the mix is that precedent from the U.S. Supreme Court is not definitive.

One of the closest times the high court came to ruling on same-sex marriage was when it declined, without comment, to hear a 1971 decision that found same-sex couples had no such rights.

The Minnesota Supreme Court had declared that marriage between a man and woman involves “the procreation and rearing of children within a family” and “is as old as the book of Genesis.”

Still, the justices have not been silent about marriage and procreation. In striking down an Oklahoma law requiring certain criminals be sterilized in 1942, the justices ruled that “marriage and procreation are fundamental to the very existence of survival of the race.”

Scholars disagree about what guidance, if any, these and other Supreme Court decisions should provide to judges today, particularly as lower courts take a fresh look at how same-sex marriage meshes with their state constitutions.

The Massachusetts high court, for example, rejected the procreation argument in 2003, ruling “the permanent commitment of the marriage partners to one another, not the begetting of children, that is the sine qua non [essential qualification] of marriage.”

Douglas Kmiec, a Pepperdine University scholar, said same-sex lawsuits are opening doors to judicial activism, whereby judges disregard high court precedent and the historical and moral value of marriage.

“To decide that traditional marriage has no rational basis in the history of the world is to make a fantastic statement of activist proportions,” he said.