President Bush says Americans should respect homosexuals, but he wants to make sure marriage is defined strictly as a union between a man and a woman.

Government lawyers are exploring measures to enshrine that definition in the law, the president said Wednesday.

"I believe in the sanctity of marriage. I believe a marriage is between a man and a woman, and I think we ought to codify that one way or the other," he said.

Still, he urged Americans not to ostracize gays.

"I am mindful that we're all sinners, and I caution those who may try to take the speck out of the neighbor's eye when they got a log in their own," the president said, invoking a biblical passage from the Gospel of St. Matthew (search).

"I think it is very important for our society to respect each individual, to welcome those with good hearts, to be a welcoming country," Bush said.

The Vatican launched a global campaign against gay marriages Thursday, warning Catholics that same-sex unions was "gravely immoral."

"There are absolutely no grounds for considering homosexual unions to be in any way similar or even remotely analogous to God's plan for marriage and family," according to its orthodoxy watchdog, the Congregation for the Doctrine of the Faith (search), in a 12-page guide approved by Pope John Paul II (search). "Marriage is holy, while homosexual acts go against the natural moral law."

Bush's remarks Wednesday offered a sop to conservatives who were angered earlier this month after he distanced himself from a House proposal for a constitutional ban on gay marriage.

Rep. Marilyn Musgrave, R-Colo., was the main sponsor of a proposal to amend the Constitution to read: "Marriage in the United States shall consist only of the union of a man and a woman." It was referred on June 25 to the House Judiciary subcommittee on the Constitution.

Musgrave wants "to let the people decide, not unelected judges who are virtually unaccountable to voters," she said Thursday on NBC's "Today."

Bush ran as a "compassionate conservative" in 2000, and is still trying to bridge the gap between his conservative base and critical swing voters. Some advisers fear any hint of intolerance will alienate middle-of-the-road Americans.

Recent polls have shown that just over half of Americans oppose gay marriage, though that opposition has been declining in recent years. A CBS-New York Times poll released Thursday found that 55 percent oppose gay marriage and 40 percent support it.

Bush's statement touched off passionate responses from groups with an interest in the issue.

"There is a real movement for same-sex marriage, and if the president doesn't intervene, and if he doesn't take leadership in this area, we could lose marriage in this country the way we know it," said Franklin Graham, president of the Billy Graham Evangelistic Association and the son of the Rev. Billy Graham. "I think the president is doing the right thing."

Gay-rights activists and a member of Congress took offense at Bush's comment that "we're all sinners," interpreting the remark as reflecting on gays and lesbians.

"While we respect President Bush's religious views, it is unbecoming of the president of the United States to characterize same-sex couples as 'sinners,"' said Matt Foreman, the National Gay and Lesbian Task Force's executive director.

Rep. Janice Schakowsky, D-Ill., wrote Bush a letter charging that he "seemed to equate homosexuality with sin," and demanding that the president apologize.

White House spokesman Scott McClellan said Bush had not meant to single out homosexuals as "sinners."

"The president doesn't believe in casting stones. He believes we ought to treat one another with dignity and respect," McClellan said.

The Human Rights Campaign, which says it is the nation's largest gay and lesbian political group, branded Bush's exploration of a law on gay marriage a "call to codify discrimination."

In 1996, President Clinton signed the Defense of Marriage Act (search), which denied federal recognition of same-sex marriages and allowed states to ignore same-sex unions licensed elsewhere. Bush's aides have said they are studying ways to strengthen the law.

"We ask the president to explain to the American people why DOMA does not already meet the objective he set this morning," the Human Rights Campaign said.

The group also pointed to a statement by Vice President Dick Cheney that suggested he had a different view than Bush's.

Asked during an October 2000 debate whether homosexuals should have all the constitutional rights enjoyed by each American citizen, Cheney said: "I don't think there should necessarily be a federal policy in this area.

"People should be free to enter into any kind of relationship they want to enter into," said Cheney. "It's really no one else's business, in terms of trying to regulate or prohibit behavior in that regard."

Cheney's daughter Mary is a lesbian.