The bishops' 47-member administrative committee said it strongly opposes "any legislative and judicial attempts, at the state and federal levels, to grant same-sex unions the equivalent status and rights of marriage -- by naming them marriage, civil unions or by other means."
The bishops' statement echoes a July 31 statement from the Vatican, which found "absolutely no grounds" for gay marriage and warned Catholic politicians that a vote in support of gay unions would be "gravely immoral."
The bishops said marriage, as understood by Christian teaching and social history, "is not just any relationship or just another institution." The prelates called heterosexual marriage "a unique, essential relationship."
However, the bishops stressed that their opposition to gay marriage -- including civil, legal partnerships such as those allowed in Vermont -- is not anti-gay.
"Our defense of marriage must focus primarily on the importance of marriage, not on homosexuality or other matters," the bishops said. "The church's teaching about the dignity of homosexual persons is clear. They must be accepted with respect, compassion and sensitivity."
Sister Mary Ann Walsh, a spokeswoman for the bishops, said the church opposes "government-sanctioned unions" but could support some limited "domestic partner" benefits that would be available to both gay and non-gay couples.
Walsh said the bishops support laws in 37 states that, similar to the 1996 federal Defense of Marriage Act, define marriage as between one man and one woman and do not recognize gay unions approved by other states.
The two-page statement represents the largest endorsement of a constitutional amendment by an American religious group. Two weeks ago (Aug. 27), the leaders of nine Orthodox Christian churches, representing about 5 million U.S. Christians, said they "cannot and will not bless same-sex unions."
The nation's largest Protestant body, the Southern Baptist Convention, passed a resolution in June "steadfastly" opposing any efforts to sanction gay marriage.
The Massachusetts Supreme Judicial Court is expected to rule within weeks on whether prohibiting gay marriage is discriminatory. Canadian officials said this summer they will legalize gay marriage, a move strongly opposed by that country's Catholic leaders.
President Bush said in July he wants to "codify" marriage as between a man and a woman, but administration officials have appeared cool to the idea of a constitutional amendment.
A July 24 survey by the Pew Forum found that 47 percent of Catholics support gay marriage, while 41 percent are opposed. Opposition has fallen 19 percentage points since 1996, the survey showed.
But last month, a Washington Post poll found declining support for gay marriage among the general population -- just 37 percent, down from 49 percent before the U.S. Supreme Court overturned sodomy laws in May.
The bishops offered only "general support" for a constitutional amendment and did not specifically endorse an amendment sponsored by Rep. Marilyn Musgrave, R-Colo., which currently has 85 co-sponsors in the House. An amendment has not been introduced in the Senate.
"The Catholic bishops' support adds to the already diverse coalition of religious groups that support the Federal Marriage Amendment, including leading African-American and Hispanic denominations, as well as Muslim, Jewish and Protestant organizations," Musgrave told Religion News Service.
Matt Daniels, who drafted the amendment as president for the Washington-based Alliance for Marriage, said "this (endorsement) is absolutely vital to the success of our cause, which is why we've labored so hard with friends like (retired Philadelphia) Cardinal (Anthony) Bevilacqua to reach this point," Daniels said.
A leading gay Catholic group criticized the bishops for the statement. Sam Sinnett, incoming president of Dignity/USA, said he was not surprised by the statement but found it "disappointing."
"What's not surprising is that they would use this anti-gay card to deflect attention from the big problems they have," Sinnett said. "They're choosing another group of people to abuse. It's incredibly sad and a terrible shame but not surprising. It just seems to be the tactic they choose."