A Harvard professor made a historian's case for same-sex marriage in federal court Tuesday, saying backers of California's Proposition 8 are offering the same rationale - that the survival of marriage is at stake - once used to defend bans on interracial unions and the legal subordination of wives.

Those who supported prohibitions on weddings across racial lines, bans dating from colonial days that the Supreme Court abolished only in 1967, often argued that "the institution would be degraded, their own marriages would be devalued" if such unions were allowed, Nancy Cott testified in San Francisco on the second day of the U.S. District Court trial of a suit challenging Prop. 8.

Similarly, she said, 19th century laws in most states that required women to surrender their property, earnings and legal status to their husbands were viewed by their supporters as "absolutely essential to what marriage was." It took a series of Supreme Court rulings in the 1970s, Cott said, to stamp out the remnants of sex discrimination in marriage laws.

Prop. 8, a November 2008 initiative, amended the California Constitution to define marriage as the union of a man and a woman, overturning a May 2008 state Supreme Court ruling that extended marital rights to gays and lesbians.

Two same-sex couples and the city of San Francisco contend the measure violates the constitutional guarantee of equal protection by discriminating on the basis of sexual orientation and gender.

Prop. 8's sponsors argue that the state is entitled to endorse traditional marriage while protecting gay rights through domestic partnerships. The plaintiffs are seeking to undermine that justification with academics starting with Cott, author of a 2000 book, "Public Vows," on the history of marriage in the United States.

Disputing assertions by the measure's defenders that marriage has been defined uniformly over time and across cultures, she said she found an "overall direction of change" in the United States toward equality within marriage and fewer restrictions on one's choice of a partner.

"Same-sex couples seem perfectly able to fulfill those roles," and allowing them to marry "would be very beneficial to the institution," Cott said. She said the movement for same-sex unions over the last 20 years has increased the social support for marriage, which hit a low ebb in the 1970s with rising divorce rates and criticism of sex roles.

In cross-examination, David Thompson, lawyer for the Prop. 8 campaign committee, sought to portray Cott as a biased advocate who had once endorsed a statement by a group advocating "alternatives to marriage," which he said included multiple partners.

Cott said she had only been endorsing the right not to marry.

Thompson also stressed the influence of Christianity on early marriage laws in the United States, part of his argument that religious and moral objections to same-sex marriage are a legitimate legal basis for Prop. 8. But Cott said U.S. marriage is a civil, secular institution, regulated by the state.

Cott was followed by George Chauncey, a Yale history professor, who described decades of discriminatory practices against homosexuals - government employee purges of the 1950s, raids on gay bars, and what he called a campaign by media, clergy and politicians from the 1930s onward to depict gay men as perverts.

Chauncey cited the 1977 campaign led by Anita Bryant to repeal a gay-rights ordinance in Dade County, Fla., who portrayed gays as predators of children. He said the Prop. 8 campaign was a more refined version of that approach.