The Civil Marriage Collaborative (CMC) was a pro-gay marriage advocacy group created by the Proteus Fund, a major center-left funding and fiscal sponsorship nonprofit. With the U.S. Supreme Court’s ruling in the 2015 case Obergefell v. Hodges, which legalized same-sex marriage nationwide, the CMC closed in 2016.
In 2004, the Proteus Fund established the Civil Marriage Collaborative (CMC) to conduct advocacy and research in support of efforts to obtain government recognition of same-sex marriages. 
As documented in the Proteus Fund’s report Hearts and Minds: The Untold Story of how Philanthropy and the Civil Marriage Collaborative Helped America Embrace Marriage Equality (archived here), the CMC began by gathering leaders of LGBT interest groups and getting them to agree on a long-term strategy for securing government recognition of same-sex marriage. The strategy would include litigation, grassroots organizing, lobbying, and electing pro-LGBT politicians. 
Another part of the strategy was to change how Americans thought about same-sex marriage. As then-CMC director Paul Di Donato (now Proteus Fund president) said, “the only way to achieve and defend a marriage equality victory nationwide was … changing the hearts and minds of Americans about the rightful place of LGBT people in our society and …why marriage matters for us.” This change would be accomplished through research and public education. 
With that plan in place, the CMC was then able to tap into its philanthropy network to fund the strategy. Over the next 11 years, the CMC directed $153 million (from left-of-center donor organizations including the Gill Foundation, Atlantic Philanthropies, Columbia Foundation, Evelyn and Walter Haas, Jr. Foundation, Open Society Foundations, and others, including anonymous donors) to LGBT groups. The CMC vetted the groups, making sure that they were effective. 
In 2015, the U.S. Supreme Court ruled in Obergefell v. Hodges that prohibiting government recognition of same-sex marriage was unconstitutional, granting CMC its preferred policy outcome. Polling also indicated CMC met its goal of securing majority support for same-sex marriage.    The Civil Marriage Collaborative closed in 2015 after the Obergefell decision. 
With the U.S. Supreme Court’s ruling in the 2015 case Obergefell v. Hodges, which legalized same-sex marriage nationwide, the CMC closed in 2016.