David J. Garrow is Research Professor of History and Law at the University of Pittsburgh.

Gay marriage is one of today's most hotly debated issues. In May, Massachusetts extended the right to marry to lesbians and gay men. On Thursday, a California court voided thousands of gay marriages performed in San Francisco. President Bush is calling for a constitutional amendment to limit marriage to male-female couples. So far such an amendment has no chance of winning congressional approval, but efforts to add anti-gay provisions to individual state constitutions are moving forward across the country.

Why is gay marriage now a front-page issue, and where did it come from? Right-wing critics blame "activist judges," but the Massachusetts court that mandated marriage equality was directly inspired by last year's historic U.S. Supreme Court ruling in Lawrence vs. Texas, which struck down the nation's few remaining state sodomy statutes and declared that gay Americans cannot be treated as second-class citizens. Conservative Justice Anthony M. Kennedy, a Republican from California and a devout Roman Catholic, wrote the opinion.

Just 17 years earlier, the high court issued an opposite, virulently homophobic decision, Bowers vs. Hardwick, which upheld the criminal statutes now voided by Lawrence. Bowers made it seem as if gays would remain the social outcasts that it claimed they always had been.

Yet over the last two decades, gays and lesbians have entered the U.S. cultural and civic mainstream. There also has been a remarkable blossoming in scholarship on gay and lesbian history, books that recaptured and brought to light a forgotten American past. One of the most notable was George Chauncey's "Gay New York," an eye-opening account of how, in the decades before 1930, gay men were openly visible and widely accepted participants in the city's social and cultural life.

Now Chauncey, a University of Chicago historian, turns his expert eye to how the evolution of gay life since then has brought the marriage issue to the fore. "Why Marriage?" is a short book, but it is a tour de force of historical analysis and explanation, essential for anyone eager to understand current political arguments.

Only with the onset of the Depression and its effect on men's status as family breadwinners, Chauncey explains, did gays encounter the widespread social ostracism and intense legal persecution that drove them underground into all-but-invisible lives until the late 1960s. "Anti-gay discrimination," he writes, "is a unique and relatively short-lived product" of the 20th century and "is neither natural nor inevitable."

Fifty years ago, gays "confronted a degree of policing and harassment that is almost unimaginable to us today" and which now "is almost entirely forgotten." David K. Johnson's "The Lavender Scare: The Cold War Persecution of Gays and Lesbians in the Federal Government" is a heart-wrenching reminder that homosexuals faced brutal employment discrimination and endless police hostility. Though the McCarthy era is remembered for the targeting of alleged Communists, far more government employees were fired for being gay than for sympathizing with the Soviet Union.

The severity of that repression, Chauncey says, stimulated the first gay political activists to speak out publicly in protest. Their initial assimilationist agenda, emphasizing that homosexuals were hard-working and patriotic Americans, not child molesters or "security risks," turned into an embrace of gay identity and pride soon after the African American freedom struggle likewise shifted from an integrationist to a cultural-pride orientation. The "coming out" of hundreds of thousands of gay Americans to their straight friends and neighbors "normalized" homosexuality, Chauncey says, by "showing outsiders that homosexuals were not so different" as they had imagined.

Beginning in the early 1980s, two tidal waves washed over gay America. The AIDS crisis "led to an unprecedented mobilization of gay men" on behalf of those who fell ill and in protest against government disinterest. About the same time, with "the astonishingly rapid appearance of what everyone soon called the lesbian baby boom," gay people emerged as two-parent families. "The mass experience of child-rearing and death," Chauncey writes, magnified both gays' visibility in society and their painful interactions with officials, who seldom treated unmarried partners with the deference accorded legal spouses....