Florida is the only state with a blanket law prohibiting homosexuals from adopting children, but the high court was told that other states could now feel free to copy the ban.
Opponents argued that the 1977 law, passed at the height of Anita Bryant's anti-homosexual campaign, was irrational because it excluded potential parents for thousands of abandoned children.
Supporters contend the state has the power to promote traditional father-mother families.
The high court's refusal to hear the case, made without comment, avoids a second showdown over gay rights there in two years. Justices, in a historic civil rights ruling, barred states in 2003 from criminalizing gay sex. The court said then that states "cannot demean their existence or control their destiny by making their private sexual conduct a crime."
The ruling set off a firestorm of criticism by conservative and religious groups. Three justices also complained that the court, generally known for its conservatism, had gone overboard in pandering to the "homosexual agenda."
The latest case involves gay foster parents in Florida who want to adopt children in their care.
The American Civil Liberties Union's Lesbian and Gay Rights Project, representing the parents, argued that that the state unconstitutionally singles out gays, based on discrimination.
"The plain and well-understood purpose of the ban was to tell gay people to go back into the closet," ACLU attorney Matthew Coles told justices in a filing.
Florida Gov. Jeb Bush has maintained that the children, often products of troubled and unstable backgrounds, should have a father and a mother.
"It is rational to believe that children need male and female influences to develop optimally, particularly in the areas of sexual and gender identity, and heterosexual role modeling," justices were told in a filing by Florida's attorney, Casey Walker.
A three-judge panel of the 11th U.S. Circuit Court of Appeals in Atlanta ruled against the men a year ago. In July, the full court declined on a 6-6 vote to reconsider the case.
"The message to the other states is you can follow Florida's lead with policies that encourage kids to be placed with moms and dads," said Mathew Staver, president of the Liberty Counsel, a conservative, Orlando, Fla.-based law group.
Opponents of the law said they were ready to combat efforts to copy it.
"Whether kids should have two moms or two dads, it's always been a fake argument. What all the professional organizations say is sexual orientation has nothing to do with whether someone is a good or bad parent," Coles said.
Florida allows gays to be foster parents, but not permanent parents.
The Child Welfare League of America had urged the Supreme Court to review the restriction and defended the parenting abilities of gays. League attorney Stuart Delery said that Florida allows singles, divorcees, people which disabilities, and even in some cases convicted criminals to adopt. The state had more than 8,000 children awaiting adoption in fiscal 2002, while there were 126,000 nationwide, Delery said.
By excluding gays, he said, "Florida ensures that many children will never have a family of their own."
The case is Lofton v. Secretary of the Florida Department of Children and Families, 04-478.
In other developments Monday, the Supreme Court: