Last week, the highest courts in two others states also dealt gay rights advocates setbacks. The New York court rejected a bid by same-sex couples to win marriage rights, and the Georgia court reinstated a constitutional amendment banning gay marriage there.
In the Nebraska case, an appeals court overturned a judge's ruling last year that the ban was too broad and deprived gays and lesbians of participation in the political process, among other things.
The amendment "and other laws limiting the state-recognized institution of marriage to heterosexual couples are rationally related to legitimate state interests and therefore do not violate the Constitution of the United States," the appeals court ruled.
Seventy percent of Nebraska voters had approved the ban in 2000.
The Nebraska amendment went farther than similar bans in many states in that it also barred same-sex couples from many legal protections afforded to heterosexual couples. For example the partners of gays and lesbians who work for the state are not entitled to share their health insurance and other benefits.
Gay-advocacy group Lambda and the ACLU's Lesbian and Gay Project sued, arguing it violated gay rights.
David Buckel, senior staff attorney at Lambda Legal, said Nebraska's ban is "the most extreme of all the anti-gay family laws in the nation" and his group is considering asking the appeals court to rehear the case.
Attorney General Jon Bruning countered that no one's freedom of expression or association was violated. Opponents "are free to gather, express themselves, lobby, and generally participate in the political process however they see fit," Bruning said. "Plaintiffs are free to petition state senators to place a constitutional amendment on the ballot. Plaintiffs are similarly free to begin an initiative process to place a constitutional amendment on the ballot, just as supporters ... did."
Forty-five states have specifically barred same-sex marriage through statutes or constitutional amendments.
Massachusetts is the only state that allows gay marriage, acquired through a court ruling, although Vermont and Connecticut allow same-sex civil unions that confer the same legal rights as heterosexual married couples.
In the Tennessee case, the American Civil Liberties Union had filed a lawsuit charging that the state failed to meet its own notification requirements for the ballot measure asking voters to ban gay marriage.
The high court ruled unanimously Friday that the ACLU did not have standing to file the suit and dismissed it.
Tennessee already has a law banning gay marriage, but lawmakers who supported the proposed amendment said they wanted a backup in case the law was overturned.