WASHINGTON — The struggle for African-Americans’ rights, symbolized by the bloody 1965 Selma march, is as old as the nation. The effort for American women’s rights began at Seneca Falls, N.Y., more than 150 years ago.

The modern fight for gay rights is, by contrast, less than a half-century old, dating from the 1969 Stonewall uprising in New York. But this week, as the Supreme Court hears two landmark cases on same-sex marriage, the speed and scope of the movement are astonishing supporters.

“We, the people, declare today that the most evident of truths — that all of us are created equal — is the star that guides us still, just as it guided our forebears through Seneca Falls, and Selma, and Stonewall,” President Obama said in his Inaugural Address in January, in a moment of history for gay men and lesbians, who were included in such a speech for the first time. “Our journey is not complete until our gay brothers and sisters are treated like anyone else under the law.”...