NEW YORK — Dr. Richard A. Isay, a psychiatrist, psychoanalyst, and gay-rights advocate who did not admit to himself that he was gay until he was 40, married, and a father, and who won a pitched battle to persuade his own profession to stop treating homosexuality as a disease, died on Thursday in Manhattan. He was 77.

The cause was cancer, said his son David. At his death, Richard Isay was a professor of psychiatry at Weill Cornell Medical College and a faculty member at the Columbia University Center for Psychoanalytic Training and Research.

‘‘He changed the way the psychoanalytic world viewed the subject of homosexuality,’’ said Dr. Jack Drescher, a training and supervising analyst at the William Alanson White Institute in New York and the author of ‘‘Psychoanalytic Therapy and the Gay Man.’’ ‘‘He was a pioneer, a very brave man. He was attacked by psychoanalysts. He took a lot of flak.’’

During the era in which Dr. Isay trained, homosexuality was seen as a ‘‘lower level of psychological development,’’ Drescher said. It was something to be cured in therapy, and openly gay professionals were barred from training as analysts at institutions accredited by the American Psychoanalytic Association, the oldest US professional group for analysts and one of the most influential, with many training institutes and affiliated societies.

Early in his career, Dr. Isay accepted the mainstream view. Troubled about his own sexuality, he thought psychoanalysis might help, and he had 10 years of therapy. In the 1970s, soon after he was supposedly ‘‘cured,’’ he realized, he said, that he was homosexual. By then, he had a wife, the former Jane Franzblau, and two sons.

For a time, he lived as a closeted gay man, but he worked with gay patients — helping them to accept themselves — and began writing about the idea that homosexuality was normal, not an illness or a matter of arrested development.

He did not tell his wife until 1980 that he was gay. They stayed married for another nine years to keep their family together, and they kept his sexuality a secret from their sons.

Dr. Isay continued to present his ideas at professional meetings, where he acknowledged that he was gay. Some colleagues stopped referring patients to him and suggested he might need more therapy.

‘‘I think he was hurt very badly by many colleagues,’’ Drescher said.

Though the American Psychiatric Association stopped classifying homosexuality as a disease in 1973, many members continued to regard it as an illness. Dr. Isay tried reasoning, badgering, and other forms of persuasion for about 15 years, but they held firm.

In 1992, backed by the American Civil Liberties Union, he threatened a lawsuit to force the association to promise not to discriminate against gay people. The group relented, issuing statements that it would not discriminate in training, hiring, or promoting analysts. In 1997, the group became the first national mental health organization to support gay marriage.

By then Dr. Isay had long had a relationship with Gordon Harrell, an artist about 20 years younger than he. They met in 1979 but did not move in together until Dr. Isay’s marriage ended in 1989. They were married last year in the living room of Dr. Isay’s son Josh. A grandson was the best man.

Dr. Richard A. Friedman, director of the psychopharmacology clinic at Weill Cornell Medical College, said that Dr. Isay had ‘‘made the field see that their view was based on ideology, not evidence.’’