(RNS) With the largest Presbyterian denomination's official endorsement Tuesday (March 17), American mainline Protestants have solidified their support for gay marriage, leaving the largest mainline denomination -- the United Methodist Church -- outside the same-sex marriage fold.

Methodists, with more than 7 million members, rejected same-sex marriage at their last national conference, in 2012. They are likely to revisit the question at their next conference, in 2016, but a growing membership in Africa, where there is little acceptance of homosexuality, makes it unlikely the denomination will accept gay marriage.

Another denomination generally considered mainline, the American Baptist Churches USA, does not allow same-sex marriage, nor do a handful of smaller mainline denominations. But the Episcopal Church, the United Church of Christ and now the Presbyterian Church (USA) sanctify the marriage of two men or two women. The 3.8 million-member Evangelical Lutheran Church in America gives congregations the autonomy to decide for themselves.

"There is no group that has moved more quickly or more dramatically on this issue than white mainline Protestants," said Dan Cox, research director of the Public Religion Research Institute, a nonprofit that studies trends in American religion.

In 2003, 36 percent of white mainline Protestants supported gay marriage, compared with 62 percent in 2014, Cox said.

And though there is not one Protestant on the Supreme Court, the fact that an increasing number of the nation's churches are inviting gay couples to the altar is likely to weigh on the justices as they consider upcoming cases that would allow them to make gay marriage a right.

Cox notes that among white mainline Protestants, Presbyterians and Methodists in the pews hold strikingly similar views on gay marriage. In that same 2014 PRRI survey, 69 percent of Presbyterians approved of same-sex marriage, while 67 percent of U.S. Methodists did.

"Support for gay marriage in these denominational families is quite strong," Cox said. "It's hard to say the churches are actually leading on this issue. They are reflecting where their followers already are."

The Rev. Jeremy Smith, minister of discipleship at First United Methodist Church in Portland, Ore., said the Presbyterian vote reminds Methodists to ask themselves why their own doctrine is the way it is.

“Why is this still on the books?” he said. “In the Methodist Church we have been behind the culture."

The majority of church-affiliated Americans belong to denominations that forbid gay marriage, including Roman Catholics, most Baptists, Pentecostals, evangelicals and Mormons.

Mainline Protestants, once the majority in America, have lost ground in recent decades to other denominations and to independent churches.

This week's Presbyterian Church vote was long expected after 61 percent of General Assembly delegates voted in June to allow gay and lesbian weddings. That made the 1.8 million-member PCUSA among the largest Christian denominations to take an embracing step toward same-sex marriage.

But the change did not become church law until a majority of the 171 regional presbyteries, or geographic regions, voted to ratify the new language. The threshold was reached Tuesday when the Palisades Presbytery in New Jersey became the 86th to approve a change in the denomination's constitution making marriage a commitment “between two people, traditionally a man and a woman.”

YS/MG END MARKOE