The lawsuit, filed in Denver District Court Wednesday by nine same-sex couples, says the ban on gay marriages is a denial of several constitutional protections, the Denver Post reported. Colorado permits civil unions between same-sex couples.
"This denial of equal protection, due process and basic fairness violates the Constitution of the United States of America," the lawsuit said.
"Colorado law creates two classes of citizens: those free to marry the person they love, and those denied that fundamental right," the suit says. "Same-sex couples in Colorado are relegated to a second-class level of citizenship that denies their relationships the full panoply of rights enjoyed by married opposite-sex couples."
While attorneys general from some states have declined to defend against similar lawsuits challenging same-sex marriage bans, Colorado Attorney General John Suthers said he is obligated to mount a defense.
"It is the job of the attorney general's office to defend our state laws, and we will defend against this new lawsuit as we would any other," Suthers said.
Gov. John Hickenlooper said in a statement that "as a matter of constitutional law, we appreciate that the courts will take it up. On the underlying question of equal rights, we believe Colorado made a step forward when we passed bipartisan civil unions legislation last year."
The Colorado lawsuit is the latest of several legal challenges of same-sex marriage bans, including challenges in Utah and Oklahoma now before the U.S. 10th Circuit Court of Appeals in Denver. The appeals court will review the federal court decisions that voided the states' constitutional bans against same-sex marriage.