"Civil marriage is a civil right and a matter of civil law," the group announced, citing the 14th Amendment's equal protection clause.
In a written statement to The Associated Press, NAACP President and CEO Benjamin Jealous said a broader coalition is needed to fight the civil rights battles of the 21st century.
"Last century we needed lawyers; this century we need big, broad coalitions," he said. "When extremists decide to attack all our communities, they must hope that there will be infighting. But we have stood all for one and one for all. That is how we will win."
For LGBT people, the fight is not yet over for the values of equality King stood for, said Darlene Nipper of the National Gay and Lesbian Task Force. Most states ban gay marriage and other civil rights for gay couples.
As a black woman and lesbian, Nipper said she will be able to bring her whole self to the 50th anniversary of the March on Washington, now that the gay rights movement is not a separate cause pushed to the side.
"It's just a powerful, palpable, beautiful progression toward the kind of society that Martin Luther King Jr. talked about when he was talking about that truly beloved community," she said. "That's really reflective of inclusion for all of us."
While Rustin was kept out of public view 50 years ago at the march, Flournoy and other gay rights advocates said they plan to speak at this year's gatherings. There will also be special events at Washington's Lincoln Theatre and at the Human Rights Campaign headquarters to honor Rustin's memory. And Rustin will be honored later this year at the White House.
"It's going to be a watershed moment," said Sharon Lettman-Hicks of the National Black Justice Coalition, which is dedicated to LGBT people of color. "But the work is not done."
Associated Press writer Suzanne Gamboa contributed to this report from Washington.
Copyright 2013 The Associated Press.