Second only to the tedious and irrelevant "But do you like him?", the prime question biographers struggle with is "Did they do it?" Between-the-sheets information is very hard to come by. But in this post-Freudian age, as Richard Holmes, doyen of the genre, has said: "It is natural to regard sexual behaviour as the ultimate intimacy, or truth of identity, with which the biographer is faced."

Lacking first-person testimony, however, or letters of the kind that James Joyce or Rupert Brooke left behind, writers can only follow their prey to the bedroom door, then ponder. The dilemma was succinctly put by the prolific biographer Jeffrey Meyers at a DH Lawrence conference in 1990. He described listening to Lady Dorothy Brett's reminiscences for hours, "when all I wanted to ask was 'Did you screw John Middleton Murry in 1919?'"

The weakness in Michael Hasting's recent play Calico lies in its unknowable base. That Joyce's daughter was infatuated with Samuel Beckett in Paris from 1928 is beyond dispute. So too is Lucia's poor mental state; she was out of control and promiscuous. Did Beckett share her favours? In a combative new biography of Lucia, Carol Shloss reluctantly concludes: "We cannot know." However, she notes that in Beckett's posthumous "Dream of Fair to Middling Women", the character Syra-Cusa appears to be modelled on Lucia and the novel's narrator "knows Syra-Cusa's body intimately". Shloss goes on to say that Joyce named his daughter for Lucia, martyr of Syracuse, patron saint of light. Hmmmm. Maybe.

In the puritanical past, sexual congress outside marriage mattered a very great deal. When, in 1943, the nuclear physicist, Robert Oppenheimer, while working on the Manhattan project to develop the atomic bomb, visited the San Francisco flat of an old girlfriend who was a known communist, secret intelligence officers kept watch across the road to witness him not leaving until morning. Maybe nothing happened. Even so, the alleged adultery with a red was used against Oppenheimer when he was later stripped of his security clearance.

Writing the biography of Rosalind Franklin, the DNA scientist who died tragically young from cancer at 37, I found no evidence of any romance. I felt obliged to ask person after person: do you think she died a virgin? Her family, polite and discreet, believed she was. But Franklin was a vigorous, well-travelled young woman who had lived for four years in Paris. Ray Gosling, her colleague at King's College, formed a different opinion: "I wouldn't have thought she 'died wondering' as my mother-in-law used to say."

It is well known that Sigmund Freud much enjoyed the company of his sister-in-law Minna and spent holidays in Italy with her. The obituaries of his grandson Walter Freud, who died in February aged 82, mentioned the family's distress at a BBC drama which had implied that Freud had had an affair with "Tante Minna": "My grandfather would have found her infinitely resistible", Walter once wrote in a newspaper article. Freud's daughter, Anna, has also been the subject of speculation. She shared her life with Mrs Dorothy Burlingham, mother of four. In their home at 20 Maresfield Gardens, Hampstead, the two women had adjoining bedrooms. But the official Freudian line is "just good friends".

The accepted version of Oscar Wilde's life is that he was not initiated into homosexuality until his mid-30s, when he was a married man. But Neil McKenna, author of The Secret Life of Oscar Wilde (2003), has found compelling evidence that Wilde was seduced much earlier. John Maynard Keynes presents the reverse case. Actively homosexual for many years, he shocked Bloomsbury by marrying the ballerina Lydia Lopokova in 1925. Robert Skidelsky, Keynes's three-volume biographer, says: "Sexual relations certainly developed, and by 1924 Lydia was appreciative of Maynard's 'subtle finger'". But was Keynes ever tempted back to gay ways? Critics of Keynesian postwar policy would dearly love to know if the great man was a law-breaker by the rules of his day.

Eleanor Roosevelt, plain, able and mocked wife of Franklin Delano Roosevelt, is now known to have written ardent letters to her dearest friend, the journalist Lorena Hickock, who responded, "I remember your eyes, with a kind of teasing smile in them, and the feeling of that soft spot just northeast of the corner of your mouth against my lips". If Mrs Roosevelt turned lesbian, however,and found her sensual self, it was after having borne six children.

The sexual predilections of political leaders raise the question: might the course of history have been different had they enjoyed more conventional satisfactions? Hitler's curious sexuality perplexed even his contemporaries. Eva Braun's maid examined the sheets after the couple's shared nights without ever finding substantial evidence. Ian Kershaw, acclaimed Hitler biographer, is more concerned by the Führer's obsession with his troubled niece Geli but whether it was "explicitly sexual cannot be known beyond doubt".

Does potency matter? WB Yeats, in his 60s, was clearly in love with the journalist Edith Shackleton Heald and spent much time at her home in Sussex. His official biographer, Roy Foster, pronounces: "While no longer capable of full intercourse, his relationship with Edith was intensely sexual: surviving blurry shapshots show her sunbathing bare-breasted in the Steyning garden under his rapturous gaze."

Similarly, the potency of Law-rence, self-proclaimed "priest of love", matters. Frail, often ill, with a promiscuous wife, in 1920 Lawrence found himself in a seductive situation in Fiesole with the beautiful Rosalind Thorneycroft Baynes. Her diaries give a detailed description of their romantic encounter: "And then to bed."

Then there are the Shaws. In our openly sexual age, the thought of a crusading social revolutionary like George Bernard Shaw retreating to abstinence is shocking. Did the couple really not do it? Never ? Shaw was on crutches from a broken leg when they married in 1898. His wife, Charlotte Payne-Townshend, was pathologically afraid of child-birth and children. Biographer Michael Holroyd finds a graceful way to suggest a negative answer: "Sex was postponed until its absence became part of the structure of their lives."

The sexual curiosity of the biographer is legitimate. The sexual act can still affect the line of royal succession and the inheritance of property. Even when nothing material is at stake, "doing it" is still an act of importance - signifying commitment or betrayal, courage or foolhardiness, or even (dare one say) love?

To smirk at the prurience of the past is condescending. The old barriers to sex were horrific. Childbirth was life-threatening, homosexuality punishable by imprisonment and, for the "guilty party" in an unhappy marriage, divorce almost unobtainable. To admire the persistence of passion in our predecessors is to recognise that "they" were just like "us".