North Carolina voters have approved a constitutional amendment defining marriage solely as a union between a man and a woman, making it the 30th state to adopt such a ban.

With most of the precincts reporting Tuesday, unofficial returns showed the amendment passing with about 61 per cent of the vote to 39 per cent against. North Carolina is the 30th state to adopt such a ban on gay marriage.

Prior to the amendment, North Carolina law already banned gay marriage, like nine other states, but the change further narrows the definition of marriage.

Legal experts in the state warn that the potential legal impact may create complications for all unmarried couples, gay or straight.

"The proposed language is problematically vague, untested, and threatens to upend years of settled law," wrote a four-person legal team from the UNC school of law.

"In prohibiting state recognition or validation of 'domestic legal unions,' the proposed Senate bill would introduce into the constitution a phrase that has never been used in any prior statutory law in North Carolina, never been interpreted by its courts, and never been interpreted by courts in any other state."

The lawyers argue that its scope is "significantly unclear" but that it might, for example prevent the courts from enforcing private agreements between unmarried couples, complicate custody agreements and invalidate domestic partnership benefits.

Tami Fitzgerald, who heads the pro-amendment group Vote FOR Marriage NC, said she believes the initiative awoke a silent majority of more active voters in the future.

"I think it sends a message to the rest of the country that marriage is between one man and one woman," Fitzgerald said at a celebration Tuesday night. "The whole point is simply that you don't rewrite the nature of God's design based on the demands of a group of adults."

Linda Toanone, who voted against the amendment, said people are born gay and it is not their choice.

"We think everybody should have the same rights as everyone else. If you're gay, lesbian, straight — whatever," she said.

Camps polarized over amendment

In the final days before the vote, members of President Barack Obama's cabinet, including Vice-President Joe Biden, expressed support for gay marriage. Former president Bill Clinton recorded phone messages urging voters to reject the amendment.

Supporters ran their own ad campaigns and church leaders urged Sunday congregations to vote for the amendment.

Both sides spent a combined $3 million on their campaigns.

With the passage of the gay marriage ban, the state would move in the opposite direction from a string of states —Democratic-leaning places such as New York and Vermont as well as conservative Iowa — where same-sex marriage is now legal.

Six states and Washington, D.C., now recognize gay unions.

'Getting married was one of the greatest things I have ever done. I hope everyone in North Carolina gets the same opportunity someday.'— Comedian Ellen DeGeneres

As voters went to the polls, several people outside the state weighed in.

"Getting married was one of the greatest things I have ever done," said comedian Ellen DeGeneres on Twitter. "I hope everyone in North Carolina gets the same opportunity someday." In 2008, DeGeneres married Portia de Rossi in their Los Angeles home.

Professional wrestler Phillip Jack Brooks, better known by his WWE ring name CM Punk, called the amendment "ridiculous."

"It's an invasion of people's rights. North Carolina, why do you care if somebody wants to marry who they love?" he tweeted.

Others were firmly in favour of the change.

"Today is the day to vote 'yes' for amendment one in North Carolina. Let's raise a standard for our nation. Protect marriage as God intended," tweeted Matthew Slyman of Charlotte, North Carolina.

Obama's campaign said Tuesday he's "disappointed" with North Carolina's constitutional amendment banning gay marriage.

Obama campaign spokesman Cameron French said in a statement that the ban on same-sex unions is "divisive and discriminatory." French says same-sex couples deserve the same rights and legal protections as straight couples.

Obama's vague stance under scrutiny

The North Carolina amendment was placed on the ballot after Republicans took over control of the state legislature after the 2010 elections, a role the party hadn't enjoyed for 140 years.

Florida, Virginia and Ohio all have constitutional amendments against gay marriage, and Obama's election-year vagueness on gay marriage has come under fresh scrutiny.

North Carolina is the latest presidential swing state to weigh in on gay marriage.

Obama, who supports most gay rights, has stopped short of backing gay marriage. Without clarification, he's said for the past year and a half that his personal views on the matter are "evolving."

Education Secretary Arne Duncan broke ranks with the White House on Monday, stating his unequivocal support for same-sex marriage one day after Biden said he is "absolutely comfortable" with same-sex married couples getting the same rights at heterosexual married couples.

'It's a generational issue. If it passes, I think it will be repealed within 20 years.'

—State house Speaker Thom Tillis

With Biden and Duncan joining the debate, Obama is likely to face renewed pressure to clarify his views ahead of the November election.

The president's aides acknowledge that Obama's position can be confusing.

In states where gay marriage already is legal, the president says married gay couples should have the same rights as married straight couples. But he does not publicly support the right of gay couples to enter into a marriage in the first place.

State house Speaker Thom Tillis, a Republican from a Charlotte suburb, said earlier in the year the amendment will likely be reversed as today's young adults age.

"It's a generational issue," Tillis told a student group at North Carolina State University in March about the amendment he supports. "If it passes, I think it will be repealed within 20 years."

With files from CBC News