“Today, he opened to the world,” she added. “To me, he opened 50 years ago.”

Ms. von Furstenberg said “we never had to talk about our relationship, we lived our relationship,” including the last five years during which Mr. Diller was writing his memoir.

“He’s been private all his life, but not with me,” she said. “So for me, it doesn’t feel strange.”

A spokesman for Mr. Diller, Paul Bogaards, said the memoir, which is being released May 20, speaks for itself.

In the excerpt in New York, Mr. Diller describes a whirlwind romance with Ms. von Furstenberg after they met in 1974, including his giving her 29 diamonds for her 29th birthday a year later. “I didn’t know what to wrap them in, so I put them in a Band-Aid box,” he writes.

The couple would later separate, before reigniting their relationship and eventually marrying at City Hall in Manhattan. (The party that followed, at Ms. von Furstenberg’s Greenwich Village home, was more glamorous, drawing guests like the designer Calvin Klein, while André Leon Talley, the famed fashion editor, said the wedding was “everything that it should be for two people who have been through thick and thin.”)

Mr. Diller, who currently serves as chairman and senior executive at the Expedia Group and at IAC, a sprawling digital media and technology company, said he had “lived for decades reading about Diane and me: about us being best friends rather than lovers.”

“We weren’t just friends. We aren’t just friends,” Mr. Diller writes, calling it “an explosion of passion that kept up for years.”

“And, yes, I also liked guys, but that was not a conflict with my love for Diane,” he said. “I can’t explain it to myself or to the world. It simply happened to both of us.”