Homosexuality is threatening a rift among US Anglicans as Church leaders meet in Minneapolis to debate the appointment of a gay bishop.
The would-be bishop said he was saddened by the dispute
The head of the Church said he was confident a split could be avoided but conservatives have threatened to walk out if the appointment is approved.
As the meeting opened, President Bush, who is a Methodist Christian, suggested that gay people should not be allowed to wed.
"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 told reporters in Washington.
The US Anglican - or Episcopal - Church faces a major showdown over homosexuality, the BBC's Jane Little reports from Minneapolis.
Laity, clergy and bishops are due to vote during the 10-day convention on whether to ratify the election of an openly gay man as bishop of New Hampshire.
If Gene Robinson is confirmed as bishop, conservatives are threatening to affiliate with supporters from Africa, Asia and the Caribbean outside the convention.
Creed and credibility
Anglicanism has traditionally been an elastic Church, tolerating a broad spectrum of beliefs, our religious affairs correspondent says.
But now the elastic holding this communion of some 80 million people worldwide - already strained by a similar crisis in the UK - looks set to snap, she notes.
However, the Episcopal head, Bishop Frank Griswold, opened Wednesday's US convention by suggesting the Holy Spirit would hold the Church together.
Gene Robinson, elected last month by churchgoers and clergy in New Hampshire, said he was saddened by threats of a split.
An agreement between Anglican bishops at the Lambeth Conference in 1998 says that homosexuals can be priests as long as they are celibate but Mr Robinson is not celibate.
The debate has focused on the interpretation of scripture, with some regarding homosexuality as an abomination.
Gays and lesbians at the convention, our correspondent says, are fighting back and challenging the church to see that the integrity and credibility of the church is at stake.
The Episcopal Church is by no means the largest US Protestant Church having a national congregation of about two million - but commentators say the convention will be followed by Christians across America.
"Everyone from Mennonites to Catholics will be watching the Episcopalians," said Martin Marty, a retired professor of Church history at the University of Chicago.
Responding to a question from journalists, Mr Bush said it was important to "respect each individual" but that did not mean he needed to "compromise on an issue like marriage".
"I think we ought to codify [marriage between a man and a woman] and we've got lawyers looking at the best way to do that," he said.
The president declined to pass moral judgment on homosexuals but commented that he was "mindful that we're all sinners".
Some Christian groups welcomed the president's remarks enthusiastically with the Reverend Pat Robertson calling for a constitutional amendment on the marriage issue.
Gay rights groups took offence at Mr Bush's "sinners" remark, interpreting it as directed at them.
"It is unbecoming of the president of the US to
characterize same-sex couples as 'sinners'," said Matt
Foreman, executive director of the National Gay and Lesbian Task Force.