(CNN) -- The Iowa Supreme Court unanimously rejected a state law Friday that banned same-sex marriage, and opponents wasted little time in pushing for a state constitutional amendment that could send the issue to voters.

Larry Hoch, left, and David Twombley, one of six couples that filed suit, celebrate Friday at a news conference.

But nothing could dampen the spirits of the six same-sex couples who shouted and clapped at a news conference held by Lambda Legal after hearing the court's opinion. Lambda Legal bills itself as the nation's oldest and largest legal group working for the civil rights of gays.

Two by two, the excited couples -- some with children -- spoke at the briefing.

"Good morning, my name is Kate Varnum, and I'd like to introduce you to my fiance, Trish Varnum. I never thought I'd be able to say that."

Dawn BarbouRoske told the group, "We're proud that our kids will be able to grow up knowing their family is respected."

Her daughter added: "Hi, I'm McKinley, and I'm really, really happy. I feel that my family has always had this right, and today it is true. No longer shall we be just people who aren't allowed to be married. We are able to get married."

Lambda Legal filed the lawsuit against Polk County's former recorder on behalf of the six same-sexcouples.

"This is a great day for civil rights in Iowa," said former Iowa Solicitor-General Dennis Johnson, a co-counsel with Lambda Legal. "Go get married. Live happily ever."

The court's decision becomes effective in 21 days, and county recorders then will be required to grant marriage licenses to same-sex couples, Lambda senior staff attorney Camilla Taylor said at the news conference.

"You do not have to be an Iowan to seek a license here," she said, and Iowa should recognize same-sex marriages from other states.

Iowa will become the third state in the nation to allow same-sex marriage, after Massachusetts and Connecticut.

Not everyone was pleased.

"It's, quite frankly, a disaster," said Brian English, a spokesman for the Iowa Family Policy Center, a nonprofit research and educational organization committed to strengthening the family.

"Obviously, we're extremely disappointed," he said. "We're saddened. Perhaps a little bit surprised in the unanimous decision that the court handed down."

English, who said opponents of gay marriage prayed outside the courthouse Friday as they awaited the court's decision, already has begun lobbying the legislature for an Iowa Marriage Amendment.

"It (the proposed amendment) will be very brief. It will reaffirm in the state constitution that marriage is the union of one man and one woman," he said. "We're beginning the next step in the process."

A spokesman for One Iowa, which supports gay and lesbian equality, said the earliest the issue could get on a ballot would be 2012.

Friday's decision upheld a 2007 ruling by a lower court that Iowa's 1998 law limiting marriage to heterosexual couples went against the state's constitution.

The state's highest court determined that "the Iowa statute limiting civil marriage to a union between a man and a woman violates the equal protection clause of the Iowa Constitution," court spokesman Steve Davis said in a written statement .Read PDF of court ruling

"The decision strikes the language from Iowa Code section 595.2 limiting civil marriage to a man and a woman. It further directs that the remaining statutory language be interpreted and applied in a manner allowing gay and lesbian people full access to the institution of civil marriage," the statement on the court's Web site says.

The Iowa Supreme Court said it has the responsibility to determine whether a law enacted by the legislative branch and enforced by the executive branch violates the Iowa Constitution.

"The court reaffirmed that a statute inconsistent with the Iowa Constitution must be declared void, even though it may be supported by strong and deep-seated traditional beliefs and popular opinion," the court said.

Polk County District Judge Robert Hanson earlier determined that the law violated the state constitution's guarantee of equal protection and hurt gay and lesbian couples "in numerous tangible and intangible" ways.

"Civil marriage in Iowa is the only gateway to an extensive legal structure that protects a married couple's relationship and family in and outside the state," Hanson ruled in Des Moines. "Iowa reserves an unparalleled array of rights, obligations and benefits to married couples and their families, privileging married couples as a financial and legal unit and stigmatizing same-sex couples."

The case was joined on appeal by several state lawmakers who opposed Hanson's ruling, calling it "a mockery of the judicial system."

They argued that the ruling stepped on the state legislature's authority by using the courts "to effectuate fundamental changes in public policies regarding marriage."

At the news conference, Jason Morgan, who was with his partner, Chuck Swaggerty, said they had been together nearly 12 years.

"We've been together in sickness and health, through the death of his mother, through the adoption of our children, through four long years of this legal battle," Morgan said, choking up. "And if being together through all of that isn't love and commitment or isn't family or isn't marriage, then I don't know what is."