But they are urging the court to resist embracing what they see as a radical change in society's view of what constitutes a marriage, especially without more information about how same-sex marriage affects children who are raised by two fathers or two mothers.
The idea that same-sex marriage might have uncertain effects on children is strongly contested by those who want the court to declare that same-sex couples have a right to marry in all 50 states. Among the 31 plaintiffs in the cases that will be argued at the court on April 28 are parents who have spent years seeking formal recognition on their children's birth certificates or adoption papers.
But opponents, in dozens of briefs asking the court to uphold state bans on same-sex marriage, insist they are not motivated by any prejudice toward gays and lesbians.
"This is an issue on which people of good will may reasonably disagree," lawyer John Bursch wrote in his defence of Michigan's gay-marriage ban. Bursch will argue on behalf of the states that same-sex couples can claim no constitutional right to marriage.
Same-sex couples now can marry in 36 states and the District of Columbia, the product of a dizzying pace of change in state marriage laws. Just three years ago, only six states allowed it.
In most states, courts have struck down gay marriage bans written into state laws or enshrined in state constitutions.
The concern for children is among several threads that run through the legal, political, social and religious arguments being advanced in support of upholding the same-sex marriage bans.
"If children don't do as well when they are raised by same-sex parents, why would we want to establish or encourage that as a social norm?" asked the Rev. D. Paul Sullins, a Catholic University sociology professor. Sullins has analyzed data in government surveys and concluded that children brought up by two parents of the same sex have a higher rate of emotional problems than their peers raised by heterosexual parents.
Sullins has been harshly criticized by sociologists who support same-sex marriage, but he said he stands by his data. "I don't know of any Catholic way to compute the equation," he said. "The idea that there are no differences is emphatically mistaken. I don't know how else to say that."
Perhaps the most visible defender of reserving marriage only for heterosexuals has been Ryan Anderson, a research fellow at the conservative Heritage Foundation.
Anderson said the push for nationwide acceptance of gay and lesbian unions is a product of cultural degradation that has spanned more than 40 years and includes greater sexual permissiveness, a rise in births to single mothers and liberalized divorce laws.
"If you think the past 40 years has been problematic for marriage," Anderson said, "you might think this is something we should go slow on and leave it to the states to make marriage policy."
But more pointedly, Anderson said states are correct in defining marriage as the union of a man and a woman because of their interest in encouraging couples to produce children and then stay together to raise those children.
"Same-sex marriage is about cutting the cord between romance and diapers," he said. "Marriage becomes just about consenting adults."
Several groups supporting the states stress that a biologically intact family is the ideal environment for children. But they also are careful to acknowledge the value of adoption, perhaps in recognition of Chief Justice Roberts' role as the father of two adopted children. "Adoption is an indispensable social good that provides children a home when the ideal, a stable, two-parent biological family, is unavailable," said a brief from groups that support biological parenting.
Another argument opponents make is that a ruling in favour of same-sex marriage could lead to opening marriage to three or more people.
"Expanding the definition of marriage away from the way cultures and civilization have always defined it can only lead to further confusion," said the Rev. Russell Moore, president of the Southern Baptist Convention' Ethics and Religious Liberty Commission. "We're always told that what we say is slippery-slope alarmism. And yet the slope is quite slippery."
These same arguments about cultural and societal harm didn't fare well when they were made in the Supreme Court two years ago in a challenge to the federal anti-gay marriage law. In an opinion written by Justice Anthony Kennedy, the court struck down the part of that law that denied federal benefits to legally married same-sex couples.
This time around, same-sex marriage opponents are drawing on the parts of Kennedy's opinion that ratified states' ability to make the rules for who could marry. They also are relying on another Kennedy opinion from last year in which the court upheld Michigan's voter-approved ban on the use of race in state college admissions.
"It is demeaning to the democratic process to presume that the voters are not capable of deciding an issue of this sensitivity on decent and rational grounds," Kennedy wrote of Michigan's affirmative action ban.
Voters in roughly 30 states, including Michigan, had approved constitutional amendments defining marriage as the union of a man and a woman.
This case, Bursch wrote on behalf of Michigan, is not about the best marriage definition, but "who decides, the people of each state or the federal judiciary?"
Moore, the Southern Baptist official, said he is under no illusion about the odds his side faces at the Supreme Court.
"We have spent several years seeking to prepare our people for the fact that the same-sex marriage seems to be coming to their communities," Moore said. "Many people in red states have assumed that this is an issue for other people."Suggest a correction