MIAMI — Six gay couples filed a lawsuit Tuesday seeking to overturn Florida’s ban on same-sex marriage, the latest in a series of cases across the country that contend such prohibitions are unconstitutional and effectively relegate gay partners to second-class status.

The lawsuit was filed in Miami-Dade Circuit Court on behalf of the couples by Equality Florida Institute Inc., a civil rights organization that works for fairness for gay, lesbian, bisexual and transgender people. The lawsuit claims Florida’s gay marriage ban violates the U.S. Constitution’s guarantees of equal protection and due process.

The couples, many of whom have children and have been together for years, said they see no reason to be forced to move to a state that permits same-sex marriage when they have built lives in Florida. Vanessa Alenier, who is raising a 5-year-old son with partner Melanie Alenier, said they decided to share the same last name to come as close to marriage as possible — but the same-sex ban blocks that final step.

“We want our son to understand that his family is secure and just as respected as any other family,” said Vanessa Alenier. “Melanie and I have worked so hard to build and protect our family, but nothing can come close to matching the protections that marriage provides.”

Similar claims have been made in other states, including Oklahoma and Utah, where judges recently struck down gay marriage bans as discriminatory. Elizabeth Schwartz, an attorney working on the Florida case, said there are about 40 lawsuits pending around the country seeking to end bans on same-sex marriage.

Florida voters enshrined a ban on same-sex marriage in the state constitution in 2008. It states that in Florida “marriage is the legal union of only one man and one woman as husband and wife,” and no other unions can be recognized.

The Florida Family Policy Council, which led the push for passage of the amendment, noted that it was adopted by 62 percent of voters that year.

“Gay activists cannot win in the marketplace, so they have resorted to trying to find renegade courts who have little respect for the rule of law to create social change that would never happen through the people or their elected representatives,” said John Stemberger, the group’s president and general counsel. “We will spend as much time and money as necessary to oppose those who seek to redefine marriage in Florida.”

The lawsuit’s advocates, however, say attitudes toward gay marriage have changed in Florida and elsewhere since 2008, with many opinion polls showing broad support for ending same-sex bans. Former Florida Gov. Charlie Crist, a Republican-turned-Democrat who is seeking is old job once again, said in a statement he backs the lawsuit.

“No one would want to be told they can’t marry the person they love. It’s an issue of fairness and I’m proud to support it,” said Crist, who hopes to challenge incumbent GOP Gov. Rick Scott in the fall election.

The law will be defended in court by the office of Attorney General Pam Bondi, a Republican. A Bondi spokeswoman said the lawsuit had not yet been reviewed.

Because they cannot marry, gay couples in Florida do not share many rights and responsibilities with opposite-sex couples. For example, gays often cannot make health care decisions for an incapacitated partner. They cannot receive worker’s compensation for a partner who dies on the job. They cannot get spousal insurance coverage and benefits. The list goes on.

“As Pam and I get older, it worries me that we do not have the legal protections that marriage provides in case one of us becomes ill or dies,” said Summer Greene, who has been with partner Pamela Faerber for 25 years.

Gay marriage bans have also been struck down by courts in New Jersey and New Mexico, and new laws have been passed elsewhere. In the seven months since the U.S. Supreme Court’s decision invalidating the federal Defense of Marriage Act, the number of states allowing gay marriage has risen from 12 to 17. Utah and Oklahoma remain pending in the courts.

Gays in Florida won a key legal victory in 2010, when the state decided not to appeal a decision striking down the state’s decades-old ban on adoptions by gay people.