Civil marriage equality in effect at the state level since October 2014. (Same-sex couples may legally marry.)
In 1975, the Boulder County Clerk issued marriage licenses to several same-sex couples after the local district attorney interpreted Colorado's statutes, which used the phrase "any two persons," to be gender-neutral with respect to marriage. The state attorney general issued a contrary opinion that those marriages were invalid. When one of the couples married in Boulder tried to use it to sponsor his husband for immigration purposes, he lost his case, Adams v. Howerton, in federal court.
Colorado is home of the historic legal case Romer v.Evans, 1993-1996. Romer v. Evans challenged CO's infamous Amendment 2, which prohibited state or local government from adopting measures that would protect gay and lesbian people as a class from discrimination. Dealing with sexual orientation and state laws, Romer was the first Supreme Court of the United States case to address gay rights since Bowers v. Hardwick in 1986, when the Court held that laws criminalizing sodomy were constitutional.
On 6 October 2014, SCOTUS declined to hear an appeal from the 10th Circuit of the ruling in Kitchen that found Utah's ban on same-sex marriage unconstitutional, leaving that decision as binding precedent in courts in Colorado. See the Burns case under LAWSUITS-PENDING below. The Pueblo County clerk began issuing marriage licenses to same-sex couples without waiting for the stay to be formally lifted.