DENVER - As lawmakers debated the idea of starting civil unions in Colorado, opponents argued that the same-sex couples would immediately condemn civil unions and seek full marriage rights, arguing that the civil unions are separate and unequal from the rights afforded straight couples.

The first civil unions have yet to be solemnized in Colorado - the law doesn't take effect until May 1 - but the prediction is already coming true.

University of Denver law professor Kris Miccio is planning a lawsuit. Not by getting a civil union, but by getting married out-of-state in a place where gay marriage is legal.

"We're going to go to Massachusetts, or New York, or if Prop 8 falls we'll go to California," Miccio said.

When the honeymoon is over, the couple will come back home to Colorado. Miccio will walk into the Denver County Clerk and Recorder's office and file her marriage certificate, which is where her legal challenge gets started.

Under the civil unions law signed by Gov. John Hickenlooper last week, her marriage from out-of-state will be transformed at this point.

"It's no longer a marriage," Miccio said. "It's downgraded to a civil union."

She uses the word downgrade for several reasons. The obvious difference between the two types of recognition has to do with the significance carried by the word "marriage" in society.

"For me, civil unions is codified second-class citizenship," Miccio said. "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."

Beyond the social significance, there are practical differences from social security to federal tax benefits.

If the US Supreme Court strikes down the federal Defense of Marriage Act, gay couples who get married in New York could file a joint tax return and potentially save a lot of money, but those couples would lose that status by moving to a state like Colorado.

"[It's] an invitation to legal chaos," 9NEWS legal analyst Scott Robinson said. "This won't be the first lawsuit proposed or filed, and I guarantee you it won't be the last."

Robinson says this sort of lawsuit has a chance, albeit a slim one.

"The problem is that our courts have typically not permitted enforcement of another state's rights and privileges created by law here," Robinson said.

Meantime, other gay rights activists are contemplating taking gay marriage to the voters again.

It's unclear whether the will of the voters has shifted that much since 2006, when Colorado enacted a constitutional same-sex marriage ban.