The media campaign will cost $250,000; its organizers said it was the largest-ever purchase of print advertising space by gay rights supporters.
"From coast to coast, millions of people and hundreds of organizations are working to protect gay and lesbian families by ending their exclusion from marriage," the ads say. "Along the way, there will be advances and setbacks, but we will not stop until every American family is treated fairly, with dignity and equality under the law."
Roberta Sklar of the National Gay and Lesbian Task Force said the ads will run in papers around the country, from The New York Times to The Gazette of Cedar Rapids, Iowa, to gay weeklies in Houston, Atlanta and San Diego.
The ads – which feature photographs of five same-sex couples who have been together as long as 53 years - are endorsed by an array of organizations and individuals, including 11 religious leaders and nine mayors.
Signatories of the ads included the mayors of Boston, Los Angeles, San Francisco, Seattle, Salt Lake City, Providence, R.I., Portland, Ore., West Sacramento, Calif., and Palm Springs, Calif.
GLAAD's executive director, Neil Giuliano, said the ad campaign was a milestone because of the strong support from straight political and religious leaders
"This clearly shows a maturing of the movement, broadening the base in our quest for equality," he said. "It shows we have the capacity to speak loudly and strongly nationwide."
The ad placement comes at an eventful time for the gay marriage debate.
Opponents of gay marriage have recently won important court rulings in several states - most notably New York. However, a proposed federal constitutional amendment to ban gay marriage failed to win the needed two-thirds support in both the Senate and House.
"There's no question the landscape is mixed," said Evan Wolfson of Freedom to Marry, one of the three organizers of the ad campaign along with the task force and the Gay and Lesbian Alliance against Defamation (GLAAD).
"This is a long-term conversation," Wolfson said. "Our job is to make sure people hear about gay families and why marriage matters, and not be drowned out by the horse race of the moment."
Among other publications, the ads were scheduled to appear in The Fresno (Calif.) Bee, The News & Observer of Raleigh, N.C., The Times of Trenton, N.J., and The Olympian of Olympia, Wash.