Give into the Zeitgeist? Not a plan of success:
Episcopal Church: - 18% (2002-2012)
United Church of Christ: - 20% (2005-2012)
Presbyterian (USA): - 22% (2006-2012)
Evangelical Lutheran - 12% (2009-2012)
Of course, the declines are likely explained by many other factors; many, if not all, these denominations were already on a downward trajectory, as their liberalizing trends didn't begin with endorsing a redefinition of marriage.
What's more, Catholics and others committed to an orthodox understanding of morality and marriage in particular should not take comfort too easily. When our Lord walked the earth, people walked away from him because of things he taught, and in the end, the crowd chanted "crucify him" instead of "my Lord and my God." So we should not expect to be popular when we offer the Lord's message.
But embracing an "evolution" of marriage didn't help, and almost certainly accelerated the decline.
Data from Alexander Griswold, "How To Shrink Your Church In One Easy Step," at The Federalist.