Just a week after civil unions were signed into law in the US state of Colorado, legal action is being planned to challenge the law in favour of equal marriage.

In a historic day for the US state of Colorado, Governor John Hickenlooper signed civil unions into law on 22 March, following several attempts to pass the legislation in recent years.

The law takes effect on 1 May, and will make Colorado the eighth state to legalise same-sex civil unions.

University of Denver law professor Kris Miccio is now planning a lawsuit to challenge civil unions, and to highlight the legal differences between equal marriage and civil unions.

Miccio said she plans to marry out of state, in one of the states where equal marriage is legal, before returning to Colorado  to file her marriage certificate.

“We’re going to go to Massachusetts, or New York, or if Prop 8 falls we’ll go to California,” she said, reports 9News.

Upon filing the marriage certificate, the Denver County Clerk and Recorder’s office will convert it to a civil union. “It’s no longer a marriage,” Miccio said. “It’s downgraded to a civil union.”

“For me, civil unions is codified second-class citizenship,” she continued. “When my daughter hears that this isn’t a real family and her mommies aren’t real mommies and that her mommies can’t get married, when she was younger, she feels less than. And that’s not healthy for children.”

Miccio’s argument weighs in on the social significant of the words used to described the union, and on top of those there are potential practical differences from social security to tax benefits.

Colorado voters passed a constitutional ban on same-sex marriage in 2006.

Some were critical of the proposed lawsuit, and said it could pass, but is unlikely to, because one state is unlikely to uphold benefits from a law in another.

The US Supreme Court Justices on Wednesday indicated a possible interest in striking down the Defense of Marriage Act (DOMA), as it heard arguments around the issue.

This was the second day of hearings, as Tuesday the court heard arguments around Proposition 8, the state of California’s ban on equal marriage. Then the justices questioned the meaning of marriage, and challenged arguments for the ban. 

A decision by the Supreme Court in both cases is expected by the end of June.