At 5:01 p.m local time, California will issue its first legally binding marriage licenses to same-sex couples. County clerks will stay open extra hours to accommodate the state supreme court’s decision to redefine marriage along these lines. The media will be filled with stories of wedding bells and pictures of happy newlyweds.

If recent experience is any guide, the number of gay couples marching to the altar will taper off in a few months even if voters uphold the California supremes’ decision in November. In Massachusetts, 1,635 such couples obtained marriage licenses in May 2004, the first month of judicially imposed same-sex nuptials. That figure, only 300 less than the number of traditional marriages in the commonwealth during the same time period, tumbled to just 148 in the first two months of 2005. Nearly 60 percent of Bay State same-sex marriages occurred in the first six months.

In Ontario, a Canadian province of nearly 12.7 million people with Massachusetts-style marriage laws, zero marriage licenses were issued to same-sex couples in the second half of 2007.

However few in number, same-sex marriage advocates will make these couples their poster children in the fall campaign against the California Marriage Protection Act. Supporters of the marriage amendment might be tempted to do so as well, showing pictures of bearded men kissing or campy commitment ceremonies in an attempt to use (declining) public revulsion against homosexuality to their benefit.

THIS TACTIC, if tried, will almost certainly backfire. Sympathy for same-sex couples and a reluctance to take benefits away from people who already enjoy them seem to outweigh any backlash against judicial activism or disapproval of homosexuality. A Boston Globe poll found that just 40 percent of Massachusetts residents favored same-sex matrimony in the spring of 2004, shortly before the supreme judicial court’s gay marriage ruling took effect. By the following March, that number had jumped 16 points to 56 percent.

After a few months of ceremony in San Francisco’s gay neighborhoods, marital traditionalists may not be able to count on the full 61.4 percent of Californians who voted against same-sex marriage in 2000.

Instead they will have to do the necessary work of proactively defending traditional marriage: explaining why it is necessary to have an institution specifically intended for the union of men and women. They must argue that the heterosexual nature of marriage is not arbitrary and not rooted in discrimination against homosexuals or anyone else, any more than limiting marriage to couples is rooted in prejudice against people from societies that practice polygamy.

Social conservatives and their allies must make this case while their opponents ask if the sky has fallen since same-sex couples began having their domestic partnerships called marriages. And they must argue forcefully that even without falling skies, redefining marriage has consequences. “Extending marriage to same-sex couples would leave no other institution to promote the ideal that every parent promises to care for his child,” as Margaret Liu McConnell put it in a recent insightful essay.

How can that be when we are so frequently reminded that many same-sex couples care for children? Because it still takes one man and one woman to create a child. Same-sex marriage will either expand multiple parenting arrangements, leading eventually to a modernized version of polygamy, or allow at least one natural parent to give up their child — through the “miracles” of modern reproductive technology, often by design.

EVEN AS WE recognize that life is complicated and children will be raised in many environments, ideally children should be cared for by both biological parents and abandoned by neither.

Hypersensitive state bureaucrats, dutifully deleting from government documents references to “husband,” “wife,” “father,” and “mother” and replacing them with gender-neutral terminology, will only add to the damage that same-sex marriage does to this ideal.

California has already attempted to offer tangible benefits to same-sex couples and meet their needs. Reducing marriage to indeterminate couplehood without biological parenthood, for the benefit of a small number of people, would not fulfill the wider community’s needs. That’s why the judgment of California’s voters eight years ago — for tolerance but against redefining marriage — was fundamentally correct.

Such an argument may not be able to compete with the initial euphoric rush on local clerks’ offices, especially in the heat of a culturally polarizing election year. But those who would reverse the California supreme court’s activist same-sex marriage decision have no choice but to make it.