Sir Arthur C. Clarke is remembered for his remarkable foresight: the sci-fi author’s works accurately predicted online shopping, biometric technology, smart phone addiction, and even targeted advertising. A 1945 Wireless World article by Clarke, who trained in radar technology during World War II, helped make communications satellites a reality. And perhaps most famously, Clarke, whose centenary occurs this month, created a whole new fearsome trope with HAL, the murderous artificial intelligence at the heart of the late author’s most famous work, 2001: A Space Odyssey.

These accomplishments are all well-known and well-celebrated among Clarke aficionados and critics. Less discussed are the ways Clarke’s works challenged heteronormative sexual mores, particularly those surrounding men who went for men. But reviewing some of Clarke’s most notable works, one sees the author surveying the changing sexual landscape of a post-Stonewall society. Taken together, they provide a panoramic view of a gay man questioning the world in which he lived.

And, yes, Clarke was a gay man, or, at the very least, queer. Though he married a woman in 1953, they separated six months later, and it’s well established that Clarke’s romantic existence was spent mostly with other men. Obsessed with the Kinsey Scale when it first came out, Clarke never believed people had strict straight or gay tendencies, a belief made clear in a number of his books. Author Michael Moorcock wrote in a 2008 Guardian essay that “everyone knew [Clarke] was gay,” even in the ’50s, well after Clarke moved to Sri Lanka, where he found the lack of sexual policing refreshing after living in uptight England. Clarke also spent 1964-1965 at New York’s famously libertine Chelsea Hotel, romping around town with Allen Ginsberg and William S. Burroughs, two of the most male-loving men of the era. And insiders also know that Clarke and a man named Leslie Ekanayake were in love; Clarke described Leslie as “the only perfect friend of a lifetime,” and the author was buried alongside him when he died in 2008.

But Clarke would never admit his love of men. Not on the record, at least. Asked by a reporter about his bedroom activities, Clarke campily laughed, “Why, what have you heard?” He only admitted his yen for men a few times: sheepishly in his semi-autobiographical 1963 novel Glide Path, in which the sexually inexperienced protagonist makes a passing reference to “a highly refined encounter with the clergyman who had (very briefly) run the local scout troop;” and off-handedly in 1986, when Playboy journalist Ken Kelley asked Clarke whether he’d had bisexual experiences. Clarke replied with a resounding yes: “Of course. Who hasn’t? Good God! If anyone had ever told me that he hadn’t, I’d have told him he was lying. But then, of course, people tend to ‘forget’ their encounters.”

He went on, “I don’t want to go into detail about my own life, but I just want it to be noted that I have a rather relaxed, sympathetic attitude about it.” Such reticence is only natural for a man born in 1917 and who came of age during the height of the Pink Scare, when western governments branded gay people as criminal scourges, as sexual criminals. And it’s equally logical that Clarke would use fiction to explore societies that had evolved past such sexual judgment.

“ Clarke’s worlds are based on logical technological leaps and real sociological change, not warp speed and wishful thinking ”

That’s the predominant theme in Clarke’s works: man’s potential for positive progress, our ability to transcend limitations self-imposed and scientific. Clarke refers to this as man’s plasticity: “One of the interesting things about the human race is its incredible plasticity. The human race can adapt to almost any circumstance.” A writer of “hard science fiction,” Clarke doesn’t create new worlds. He believed true science fiction was “something which we would like to happen and it probably will.” Clarke’s worlds are based on logical technological leaps and real sociological change, not warp speed and wishful thinking, and he therefore used his works to predict how humankind could evolve. Yet while Clarke vividly sees mankind moving past its dependence on oil, overcoming religious strife, and even tackling interplanetary space travel, he’s less certain about how we’ll conquer our sexual hang-ups.

This interest in progressive sexual mores is clear in his first commercial success, 1954’s Childhood’s End. Most of the plot is propelled by an alien race’s arrival on earth, but Clarke goes out of his way to mention that sexual etiquette had been “altered radically” by the invention of fast and cheap DNA paternity tests and by the arrival of a “completely reliable oral contraceptive.” Combined, these inventions “swept away the last remnants of the Puritan aberration.” Again, this was 1954: just as the pill was first being explored and years ahead of actual DNA testing. Premonitory as always, Clarke saw the first hints of sexual revolution coming into focus. And he liked what he saw. And though Childhood’s End makes no reference to same-sex sex, the subject definitely arises aplenty in Clarke’s future novels, including 1972’s Rendezvous with Rama.

Clarke’s first novel after 2001: A Space Odyssey and its film adaptation launched his career, Rama was also his first post-Stonewall novel, and the changing climate is evident: In Rama’s future, 2130, polyamory and same-sex love are standard, totally accepted fare: Two astronauts, Karl Mercer and his “inseparable companion” Joe Calvert, not only share a wife, but their own sexual relationship, as well: “No one can predict where lightning will strike, and years ago Mercer and Calvert had established an apparently stable liaison. That was common enough.” Clarke seems more sanguine about social change afoot, more confident in predicting a judgment free future. That optimism, however, must have been short-lived, because there’s far less nonchalance about same-sex love in Clarke’s next novel, Imperial Earth.

Published in 1976, just after Margaret Thatcher took the helm of Britain’s Conservative Party, this novel posits a future that is indeed more progressive than our present—20th-century monogamy is likened to “sexual possessiveness”—but old-school shaming over same-sex love remains in full effect, as seen in the relationship between homosexually-inclined protagonist Duncan Mackenzie and his more questioning but flexible frenemy, Karl Helmers. Scions of two prominent families, the men fooled around in their teens and into their twenties. Duncan describes their encounters: “[Karl’s] lovemaking often lacked tenderness and consideration;” and the sexual tension followed them into adulthood, as when Karl made Duncan stay in bed with him while he had sex with a woman (“They enjoyed having me there, just to tease me. Or at least Karl did.”)

Although Duncan is all-in on this lusty affair, Karl is less confident: He has a breakdown soon after that “threesome” and returns from an asylum a broken man. (“He was as beautiful as ever—perhaps even more so … and he could still be friendly … But real communication was missing.”). The men keep it straight and narrow from then on, Duncan in large part because of his overbearing grandfather’s moralistic pep talk: If Duncan succeeds in business, “no one will criticize any of your other activities, public or private.” Duncan doesn’t believe in “polarized” sexuality, he’s forced into the closet just the same. Clarke was obviously still unsure whether society would ever evolve to the future for which he longed, and Karl’s breakdown is the author’s way of illustrating how top-down sexual limitations take their toll.

Clarke’s view of sexual acceptance was somewhat rosier come 1982, the year he published 2010, the sequel to 2001 that features Soviet lovers Walter Curnow, a bisexual bear, and a young, devoted twink, Maxim Brajlovsky. Initially this relationship is only alluded to, but it’s made perfectly clear when American astronaut Heywood Floyd raises his busy-body concerns: “There’s a personal matter I’d like to raise,” Floyd says. “To be blunt, your behavior with Max.” Unperturbed, Curnow replies, “in a soft yet implacable voice: ‘I was under the distinct impression that he was more than eighteen.’” There’s no room for debate here. The American should take his moralism and shove it. And he does—and it does him some good: Later, Heywood wonders why he questioned Walter in the first place: “Floyd sometimes ruefully suspected [that] it was no more than the secret envy that normal homo or heterosexuals feel, if completely honest with themselves, toward cheerfully well-adjusted polymorphs.” But Floyd eventually evolves past his own issues: In 2061, the 1987 threequel in which he lives next to George and Jerry, the two old queens are described as the astronaut’s “oldest and closest friends.” And, as in 2010, Floyd admires this uncomplicated male-on-male love: “[Floyd] often envied the long-term stability of their relationship, apparently quite unaffected by the ‘nephews’ from Earth or Moon who visited them from time to time.”

Clarke is clearly commenting on many gay couples’ lack of sexual possessiveness. (Some readers even believe 2001 super-computer HAL went psycho because it was jealous of spaceman-turned-starman Dave Bowman and fellow astronaut Frank Poole’s burgeoning love, a theory somewhat bolstered by the fact that HAL and Bowman later fuse into one being, as well as by Poole’s memory of himself and Bowman talking about their lack of sex drive in space, a somewhat odd thing for two “straight” men to be missing while in space together, just the two of them.)

Elsewhere, George later says that a female writer’s work is “too feminist.” It’s unclear if Clarke is perpetuating the myth that gay men are anti-feminist, or poking fun at it. But he is definitely challenging social norms about gay men when George offers a litany of gay warriors in history: “We did know about Hadrian and Alexander … Richard the Lion Heart and Saladin … Julius Caesar—though he was everything … and Frederick the Great.” Clarke revisited this topic again in a satirical late ’90s essay, “The Gay Warlords,” in which he describes Caesar’s “ambisexterousness.”

The author’s personal feelings on—or hopes for—human sexuality are perhaps most clear in his 1986 novel The Songs of Distant Earth. His sexiest work—almost every character is bed-hopping with another, or hoping to—Songs lays this society’s feeling out in the open with this exchange between two men at a hospital: Lieutenant Horton explains to his roommate, Loren Lorenson, that he was injured during a surfing expedition with a group of “hairy hunks” known for their homo-social ways. Loren is surprised by the revelation: “I’d have sworn you were ninety percent hetero.” Horton replies, “Ninety-two, according to my profile, but I like to check my calibration from time to time.” This prompts Loren to recall that “he had heard that hundred percenters were so rare that they were classed as pathological.” Clarke’s old interest in Kinsey’s work remained unabated. His only hope was the rest of humanity would see things as he did.

Clarke died in 2008, the same year conservatives used Proposition 8 to beat back marriage equality in California. He never lived to see the Supreme Court rule in favor of love. Nor did he see the same wave of progress sweep England, Australia, Brazil, France, and so many other lands. Today, almost a decade after Clarke’s death, millions of people live in a world in which marriage equality is a reality, in which transgender people are increasingly accepted and in which heteronormative notions of love and sexuality are steadily eroding, even though this brave new world of acceptance remains tenuous, at best.

The world of today is still a far cry from the multiple futures Clarke imagined. But if he were here today, he would no doubt tell us to keep up the fight, because, a few utterly impossible dreams aside, like fantastical space elevators, Clarke believed the futures his works described were attainable. He was, he said in 1970, a fundamental optimist: “I believe that the future is not predetermined, that to some extent we can determine our own destiny. By thinking about the future and its possibility, we do have a chance of averting the more disastrous one.”