The conventional wisdom among the secular left is that churches must embrace gay-marriage (as well as abortion, redistribution of wealth, gun control, global warming, and all of those other things the secular left is gung-ho for) or they will perish because they aren’t cool enough to attract hip, socially-conscious secular leftists to their denominations.
In the real world, though, it’s not working out that way. Since the Episcopal Church became the pro-gay, pro-socialism, and all the rest, its membership has fallen almost 19%. The Evangelical Lutheran Church embraced gay marriage in 2009, and its membership has taken a similar tumble and it has lost over 600 congregations. The United Church of Christ — home of the Reverend Jeremiah Wright and a bastion of bat-[stuff] crazy leftism — embraced gay marriage full-bore and has been declining rapidly, 20.5% gone and still losing members. The Presbyterian Church has lost more than 20% of its members since deciding to ordain gay and lesbian clergy.
Meanwhile, conservative evangelical congregations and Mormons (whom the Secular Left would consider ‘hardcore’) are continuing to grow.
The secular left doesn’t understand religion. To the secular left, religion is a hobby (and a silly one, at best) and churches are just social clubs. The idea that there is Eternal Truth, and that it is the role of churches to protect and preserve it, is utterly alien to people whose concept of truth is built around “it depends on what the definition of ‘is’ is.” One thing Christians understand that leftist don’t is you can follow God’s word, or you can be popular and worldly. You can’t do both.
Secular leftists don’t want to go to church; in fact, they want to destroy religion because it’s a threat to the state. Churches that follow the advice of people who want to destroy them get what they deserve. [See also, Republicans and Illegal Immigration.]