One morning in Hampstead in 1944, Thom Gunn, aged 15, came downstairs to find his dead mother slumped on the kitchen floor in front of the oven, having gassed herself. Her husband had deserted her, as had her current lover; a note she left for her two young sons addressed them as “my darlings” and told them to fetch the charlady. “I was made to feel too much for my age,” Gunn later complained when remembering that day, and he was left with a lifelong dread of “deep emotion”. When in 1991 he finally wrote a poem about the trauma, he took care, as he said, “to objectify the situation”. He described it as if it had happened to someone else, and despite the need to find a rhyme for “other”, he refused to use the painful, accusatory word “mother”.

As Michael Nott points out in his fine, frank biography, Gunn’s writing developed as an exercise in denial, a defence against the insecurity and sense of abandonment he felt from that moment on. He donned masks, struck poses and emulated heroes who did not suffer from his own “fragile selfhood”. Shakespeare’s brawny Coriolanus, who boasts of being the “author of himself”, was one of his early infatuations, followed, after his move to the US in 1954, by embodiments of machismo from popular culture: Brando astride a motorbike in The Wild One, Elvis wielding a guitar as a weapon and mobilising his pelvis in a “posture for combat”.

Settling in San Francisco, Gunn found his tribe in gay bars such as the Stud or the Tool Box, where the sullen, leather-clad habitués concocted selves, as he said, to go with their impervious kit. Gunn acquired the gear and had a black panther tattooed on his forearm in the hope that the virile insignia would toughen him and make him immune to pain. In one set of ithyphallic poems he claimed to be a centaur, human from the waist up with an equine lower half. “I am raw meat,” he wrote as he offered himself for consumption in a sweaty sauna; he went on to declare: “I am a god”. But in this mock proletarian carnal jungle, his skinny, gangly physique and his nerdy English temperament let him down. “Too mousy, alas,” he moaned when conceding that he would never debauch himself as wholeheartedly as Baudelaire and Rimbaud had done; Nott tartly judges that he was “butch in costume only”.

A duplex in the hippy enclave of Haight-Ashbury strained to accommodate the unstable commune Gunn formed. Its anchorage was his maritally devoted relationship with Mike Kitaj, who represented his Platonic idea of “the American Boy”, a crew-cut figure from a comic book; but Mike was serially monogamous while, for Thom, gay liberation meant unremitting promiscuity, so they didn’t sleep together. Overnight tricks and longer-term lovers therefore came and went, together with drug dealers and, on one occasion, a raiding party of narcs who impounded a marijuana plantation and a cache of 5,000 speed pills.

Having addictive sex with multiple partners counted, Gunn thought, as “an entrance into all humanity”, though a “brute thrust” into bodily cavities was no substitute for genuine fellow-feeling. The embrace also excluded the half of humanity to which his mother belonged. Women only mattered, he joked, if “they cook us cakes”. Aids enjoined a rethink: Gunn reported that he and his so-called “fuckbuddies” took to “prodding each other with broomsticks held in welding gloves”. (This, I think, is a metaphor, though the elaborate protocols of fetishism mean that you can never tell.) More soberly, the health crisis made him realise that the quest for euphoria could be suicidal, and in his tender elegies for stricken friends he finally recognised that “sex, friendship and love are mutually inclusive”. He apologised for his own escape from Aids: he was saved, he winced, by the amoral “animal vigour” that kept him going after he lost his mother.

As he aged, still compulsively on the pull, he found himself ironically taking the place of the parents who had failed him. The hustlers he brought home regarded him as a sugar daddy because he supplied them with drugs, and as he petted a favourite young ruffian he sighed: “I’m like an old mother.” One weekend in 2004, his housemates heard him let in a nocturnal visitor, then didn’t see him again. They assumed he was in bed “fucked up and fucking”, but when they investigated on Sunday night they found – in a bleak repetition of the scene in the Hampstead kitchen – that he was alone and had been dead for hours. A heart attack was said to be the cause; in fact, he had been injected with a brew of heroin and methamphetamine.

The subtitle of Nott’s book proposes that Gunn had “a cool queer life”. Proudly queer he certainly was: he once scandalised the members of a stuffy Pall Mall club by arriving for lunch with his publisher dressed in his leather cruising regalia, complete with cowboy chaps, and he celebrated a gay street fair in San Francisco as an Arcadian panorama, a “Masque of Difference and Likeness” that he compared with the allegorical revels staged at Renaissance courts. But cool? I’d have said that Gunn stayed permanently febrile, more or less until the end. Even so, sensual excitement, intensified by chemicals, was always a prelude to the chilly, desensitised numbness he craved, and during that last lost weekend he attained degree zero.