The New York Times has posthumously outed Ed Koch, the three-time Democrat mayor of New York City from 1978 to 1989, as gay. The piece is a detailed account of the loneliness of a closeted politician who entered politics before 1973, the year the American Psychiatric Association stopped classifying homosexuality as a mental illness. It might not be news – Koch’s gayness was the worst-kept secret in New York politics ­– but it is history.

Koch, who died in 2013 aged 88, was the son of immigrants from Galicia. Calling himself a “liberal with sanity”, he revived NYC after the fiscal crisis of the early Seventies and rebuilt the city’s public housing. He was the liberal Jewish face of New York when liberal Jews defined the city and its politics. But he was also a coalition-builder in a city of sharp-elbowed ethnic rivalry.

In 1977, Koch fought his first mayoral campaign against Mario Cuomo, a fellow Democrat from the outer borough of Queens. “Vote for Cuomo, not the homo,” read the posters that appeared all over Queens. The Cuomo campaign denied responsibility. Koch beat Cuomo for the mayor’s job, but in 1982, Cuomo beat Koch in the race for governor of New York State. In 1981, Koch won re-election with 75 per cent of the vote and the endorsement of both parties – then raised his share of the vote to 78 per cent in 1985. He was the congressman from Greenwich Village, the mayor of one of the most liberal cities in the country, and he came from the progressive wing of the Democrats, but none of this protected him against innuendo.

“Homosexuals were seen as a threat to national security, and homophobia was a weapon that everyone used,” says James Kirchick, author of a new and groundbreaking history, Secret City: The Hidden History of Gay Washington. “The left used it against the right, the right used it against the left. This notion that homosexuality and gay rights are a progressive issue, that really didn’t happen until the Eighties or the Nineties even.”

Koch’s reputation might be a victim of his success: he failed to navigate from one era to another, more assertive one, and his third term turned bitter. Race relations deteriorated – in 1988, he excoriated Jesse Jackson for calling New York “Hymietown”. His pretend romance with Bess Myerson, who had been the first Jewish woman to win Miss America, misfired in a corruption scandal. And though he pushed an ordinance protecting gay and lesbian rights through the City Council, he was slow to respond to the AIDS crisis – because, some activists claimed, he had something to hide. “Koch’s story shows the pain that an individual in the closet had to live with, but also how it affected public policy,” Kirchick says.

“Being successful increases that pressure of being in the closet: you convince yourself that I have this success because I’ve kept this thing about me a secret. So you sort of believe your own kind of propaganda. But who knows what would have happened if Koch had come out of the closet?”

The story goes that Koch lived in the same building as the playwright and activist Larry Kramer, a co-founder of ACT UP, the coalition to raise public awareness of AIDS. Kramer hassled Koch so much that Koch secured a court order banning Kramer from speaking to him when they passed in the lobby. So Kramer resorted to loudly telling his dog: “That’s the man who killed all of Daddy’s friends.”

Ed Koch’s sexuality was his own business when he was alive, but, Kirchick argues, after his death, it’s “all of our business, especially if it matters to what he did in public life”. But Kirchick also warns against forgetting what put Ed Koch and many other politicians in the closet in the first place.

“Straight people were the ones who created the environment of hysterical homophobia that ruled over American politics for decades. You can fault Ed Koch as much as you want for not having personal courage, or foresight, or the right policies.

“But he was ultimately a gay man growing up in a world where his very nature was a threat to the country. You have to put yourself in someone’s shoes to understand the kind of choices they had to make.”

James Kirchick’s “Secret City: The Hidden History of Gay Washington” (Macmillan).

Dominic Green is a British historian based in Boston