When Oliver Sacks was 18, he faced a prospect most young people dread: a belated talk about the birds and the bees with his dad.

“You don’t seem to have many girlfriends,” Sacks wrote his father said in his memoir, “On the Move,” released earlier this year. “Don’t you like girls? … Perhaps you prefer boys?”

Sacks didn’t try to hide.

“Yes I do – but it’s just a feeling – I have never ‘done’ anything,” Sacks told his father.

He pleaded with his father not to tell his mother – but his father did. The news did not go over well — to say the least.

“You are an abomination,” she said. “I wish you had never been born.”

Sacks wrote that his mother’s words had to be understood in the context of the times. Homosexual acts were not decriminalized in England until the 1960s; his mother, he wrote, “had an Orthodox upbringing.” Yet, her denunciation would prove crushing to a young man about to embark on a brilliant career as a neurologist.

“Her words haunted me for much of my life and played a major part in inhibiting and injecting with guilt what should have been a free and joyous expression of sexuality,” he wrote.

When Sacks died this week at 82, he was memorialized as a brilliant doctor and author — the man immortalized by Robin Williams in the film “Awakenings.” Yet, many tributes gave short shrift to an astonishing fact about a man who seemed so empathetic when trying to puzzle through his patients’ most debilitating neurological afflictions and open about his own depression and drug use: He lived in self-imposed celibacy for more than three decades, only coming out in the past few months. And it didn’t sound all that fun.

References to Sacks’s “celibacy” crop up in news stories in the mid-1990s after the publication of his 1995 book “An Anthropologist on Mars” — with no reference to Sacks’s sexual orientation. Some of Sacks’s comments are, more or less, heartbreaking.

“I’ve been alone and celibate for so long I can’t imagine it otherwise,” he told the Daily Mail in 1995.

“He is still unmarried (and celibate) at 61 and says he is not likely to change,” London’s Evening Standard wrote that same year. “He visits his analyst twice a week and the baby gorillas in Central Park zoo daily.”

Sacks’s abstention was practically part of his authorial image.

“Diffident, celibate, disastrously absent-minded and accident-prone; a hippy who was into drugs, bikes and body-building in 1960s California; now a professor of neurology in New York,” the Guardian wrote in 2001. “Many of his readers must have wondered what makes him tick.”

This logjam seemed to break just three years ago. In Sacks’s 2012 book “Hallucinations,” there was a passing reference to a decades-old “a love affair … gone sour.” New York magazine called this “what might be the first reference in his work to his own romantic life.”

Sacks also offered another mournful quote to the magazine.

“I lived alone, I’ve always lived alone,” he said. “I am, I believe the phrase is — one speaks of people as being ‘married to their work.’ ”

One friendly reader thought this strange. It seemed like Sacks was gay. Why wasn’t he saying so?

“Oliver Sacks is a remarkably appealing and admirable figure,” the commenter wrote. “It seems impossible to ignore the obvious implication conveyed by the description of his history (Thom Gunn, motorcycles, leather, San Francisco sexual awakening) that he is gay, if celibate in later years. I wonder why this was not touched on explicitly.”

Yet it wasn’t until this year, as Sacks faced a terminal cancer diagnosis, that his memoir appeared and the truth came out. His celibacy had ended when, at 77, he began a relationship with author Bill Hayes, described in passing in his New York Times obituary as “his partner of six years.”

When writing about this late love affair in “On the Move,” Sacks seemed happy — and ready to make up for lost time in the little time that remained.

“It has sometimes seemed to me that I have lived at a certain distance from life,” he wrote. “This changed when Billy and I fell in love.”