By Keith Coffman
DENVER (Reuters) - Nine same-sex couples filed suit on Wednesday challenging Colorado's ban on gay marriage, following a recent string of court decisions striking down similar restrictions in New Mexico, Utah and Virginia.
The lawsuit, filed in state court in Denver, says a 2006 voter-passed state constitutional amendment defining marriage as exclusively "a union of one man and one woman" violates U.S. constitutional guarantees of equal protection and due process.
The suit also takes aim at a civil unions law enacted by the Democratic-controlled state Legislature last year, saying it fails to grant gay and lesbian couples legal status equal to heterosexual couples.
"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," the lawsuit said.
"Even same-sex couples who have been validly married in other states are stripped of their marital status when they enter the state of Colorado," according to the 24-page complaint.
Five of the couples named as plaintiffs in the lawsuit are unmarried but are in committed relationships and wish to marry. The four others have been legally wed in other states and their marriages have been reduced to civil unions at home in Colorado.
The lawsuit names Governor John Hickenlooper, a Democrat, and Denver's county clerk as defendants.
Colorado Attorney General John Suthers, a Republican, said his office would defend the gay marriage ban in court, adding, "This is an important issue that the appellate courts will ultimately resolve."
The case in Colorado, a Democratic-leaning state but which also has a conservative political streak, comes at a time of growing momentum toward legalizing gay marriage in the courts, at the ballot box and in statehouses across the country.
Since mid-December, gay rights activists have won a series of court battles in New Mexico, Utah and Virginia, where prohibitions on same-sex marriage were ruled unconstitutional by federal judges. The Utah and Virginia decisions have been stayed pending appeal.
And a federal judge in Kentucky ruled last week that marriages of same-sex state residents who were legally wed elsewhere must be recognized in their home state.
In all, 17 states plus the District of Columbia recognize gay marriage, including eight states where it became legal in 2013.
The trend has gained steam since the U.S. Supreme Court in June ruled that legally married same-sex couples nationwide are eligible for federal benefits, striking down a key part of the 1996 federal Defense of Marriage Act.
That decision, and a separate ruling paving the way for gay marriage to resume in California, stopped short of extending a constitutional right to marry to same-sex couples.
Some legal experts have said the recent flurry of federal court decisions in favor of gay marriage may hasten Supreme Court consideration of a nationwide right to same-sex nuptials.
More than 30 states still ban gay or lesbian couples from marrying, by constitutional amendment, by statute, or both.
(Reporting by Keith Coffman; Editing by Steve Gorman, Cynthia Osterman and Ken Wills)