The Presbyterian Church (USA) have voted to change the definition of marriage from "one man and one woman" to "two people, traditionally a man and a woman," thus clearing the way for clergy to perform same-sex marriages.

No member of the clergy will be forced to perform gay marriages if personally opposed.

New York Times:

“Finally, the church in its constitutional documents fully recognizes that the love of gays and lesbian couples is worth celebrating in the faith community,” said the Rev. Brian D. Ellison, executive director of the Covenant Network of Presbyterians, which advocates gay inclusion in the church. “There is still disagreement, and I don’t mean to minimize that, but I think we are learning that we can disagree and still be church together.”

The church, with about 1.8 million members, is the largest of the nation’s Presbyterian denominations, but it has been losing congregations and individual members as it has moved to the left theologically over the past several years. There was a wave of departures in and after 2011, when the presbyteries ratified a decision to ordain gays and lesbians as pastors, elders and deacons, and that may have cleared the way for Tuesday’s vote.

With many conservative Presbyterians who were active in the church now gone, as well as the larger cultural shift toward acceptance of same-sex marriage, the decisive vote moved quickly toward approval, according to those on both sides of the divide.

Plenty of moderates and conservatives, however, have chosen to stay within the Presbyterian Church (U.S.A.), one of the nation’s historic mainline Protestant denominations, which has its headquarters in Louisville, Ky. Ministers who object will not be required to perform a same-sex marriage.

Paul Detterman, national director of The Fellowship Community, a group of conservatives who have stayed in the church, said: “Our objection to the passage of the marriage amendment is in no way, shape or form anti-gay. It is in no way intended as anything but concern that the church is capitulating to the culture and is misrepresenting the message of Scripture.”

He added, “We definitely will see another wave, a sizable wave, of conservative folks leaving,” but said he and others were staying because “this conversation is dreadfully important to be a part of.”

PC USA is one of the largest mainline Protestant denominations in the country, but it has been bleeding members since the 2011 decision to allow gay clergy.

The departures, mostly for the Evangelical Presbyterian Church and ECO: A Covenant Order of Evangelical Presbyterians, have been ongoing for some time now, but the cumulative impact is adding up. ECO reports that 60 congregations so far have formally joined its ranks, with more either in discernment or on the way.

The PC(USA) reported that in 2012, the denomination dismissed 110 congregations to other denominations, compared to 21 in 2011. Another 86 congregations were dissolved ― typically, too small to continue operating ― and overall, the PC(USA) lost more than 102,000 members in 2012, according to the denomination’s 2012 comparative statistics report.

That 5 percent membership loss constituted the denomination’s net numerical loss since the 1970s and largest loss, in percentage terms, in almost 50 years of ongoing membership declines. The PC(USA) now reports 1.84 million members, less than half of its peak membership of 4.25 million members in 1965 and down from 1.95 million members in 2011.

Some of the potential departures have been low on drama, particularly in presbyteries with gracious separation policies. Others have involved lawsuits over who owns the property ― such as the one Highland Park Presbyterian Church, with nearly 4,900 members, filed Sept. 10 in Dallas County District Court against Grace Presbytery.

All denominations have been losing members in recent decades, but the PC USA lurch to the left appears to have driven away a fair number of conservatives.  This vote was a foregone conclusion, given that the conservative opposition has largely pulled out, clearing the way for domination by church liberals.

The PC USA has always been one of the more liberal denominations in the U.S., so the vote to allow clergy to perform same-sex marriages is hardly surprising.  As this chart shows, most mainline denominations still oppose gay marriage, although there are varying degrees of activism in all denominations to allow gay marriages.

That chart will look vastly different ten years from now.