Sex, libel and eugenics ...

The Edwardian who was sex therapist to a nation and a pioneer of selective breeding was, privately, a jealous mother. Marie Stopes's son, Harry, spoke to Emma Brooker; 'Marie was like a snake ... I honestly thought she might poison us both, but I didn't want to become involved in a battle for her son. I was much too frightened'

Marie Stopes is remembered as one of the great liberators of British women. Author of the first sex manual, advocate of women's right to enjoy sex and founder of the nation's birth control clinics, she improved the lives of subsequent generations of women beyond measure. But Stopes's campaign for the widespread availability of birth control was underpinned by an interest in the "perfection of the race" through selective breeding - ideas that are uncomfortably redolent of those pursued by Hitler. The gap between Stopes's public image and her personal actions was, on occasions, very wide, as her son, Dr Harry Stopes Roe explains, nearly 40 years after her death in 1958.

A retired philosophy lecturer living in the Birmingham suburb of Moseley, Dr Stopes Roe bears a striking resemblance to his fiery mother, whose intense eyes and strong-set jaw he has inherited. He is immensely proud of what she achieved through her groundbreaking sex manuals and birth control clinics, but both Harry, now 71, and his wife Mary still flinch when they describe how Marie received the news of their engagement.

"I thought she was going to kill us," says Mary, herself a doctor of psychology and daughter of another popular legend, Barnes Wallace, the inventor of the bouncing-bomb. "I honestly thought she might poison us both, but I didn't want to become involved in a battle for him. I was much too frightened." It was the start of a campaign against their relationship that Marie was to wage to her grave and beyond, largely cutting Harry and his family out of her will.

The cause of offence, astonishingly, was Mary's mild short-sightedness. The fact that Harry's betrothed wore glasses showed a genetic deficiency which, Marie claimed, would be passed on to her own descendants. "On eugenic grounds, I should advise against the marriage were they strangers to me," the world-famous campaigner for equal rights wrote to Barnes Wallace in 1947. To her husband she complained the marriage would "make a mock of our lives' work for eugenic breeding and the race," and suggested that their grandchildren would be burdened with "defective sight and the handicap of goggles".

"She was like a snake," says Mary under her breath. Harry is more forgiving in his interpretation of his mother's fury. "It was emotional, she was finding a pretext. She believed that Mary and I would not make a happy marriage and was rationalising what she felt. I think any woman displacing her would have had to have a very special relationship to her to have been acceptable."

Stopes's jealous reaction to her son's engagement betrayed the fact that her own marriage was, privately, a failure; another of the contradictions in her life. The publication in 1918 of her book Married Love had set Edwardian society on fire with its controversial suggestion that women could, and should, enjoy sex within marriage. Stopes became marriage guidance counsellor and sex therapist to the nation, advising millions of readers on how often they should copulate - "three or four days of repeated unions followed by 10 days without any unions at all" - and recommending an aphrodisiac diet of oranges, eggs, whitebait and oysters.

Stopes was a virgin when she wrote the book - her first marriage was unconsummated - and her subsequent marriage to Humphrey Roe was physically and emotionally disastrous. While Humphrey supplied the initial idea and funding for the first birth control clinics, he got little credit for it. In 1938, after 20 years of marriage, Humphrey, who adored his wife to the end, signed a desperately pathetic note, which may have been dictated by Stopes, permitting her to take a lover. "I have long considered a wife whose husband is incapable of coitus has every right to supplement his deficiency."

Rarely allowed to visit Norbury Park, the grand family home in Surrey, Humphrey took lodgings in London, while Harry, or "Buffkins", whom Stopes described as "celestially beautiful" seems to have taken her husband's place in her affections. "We were very close indeed, I think one might say too close," says Harry with a smile. "Certainly, I was a very important factor in her life. I think she saw me as a perfect child."

Stopes would regale visitors with stories of the four-year-old Harry conducting scientific research as he played on the lawn. Harry was not taught to read until the age of eight because his mother wanted him to learn to think for himself. Dressed in knitted kilts and trousers up to the age of 15, to prevent genital damage (that Marie believed ordinary trousers could cause), Harry's childhood was, in some ways, intensely pressurised.

But Harry totally reciprocated his mother's doting love. "Despite all the pressure on her, she devoted a lot of time to me," he says. Stopes brought a series of libel actions against those who attacked her work, but at home she rarely mentioned the fierce hostility towards her. "I think she deliberately protected me from it," says Harry.

Stopes became obsessed with what she believed was a harassment campaign mounted by Catholics. The Times boycotted any mention of her for years because, Stopes believed, of the work of Catholic members of staff there. And when a Catholic woman burnt down one of the clinics - "egged on by her priest", says Harry - Stopes began to see a conspiracy. "I think one must acknowledge that the persecution pushed her over, and it is one of the reasons, one might even say justifications for her excessiveness at times and her megalomania."

It was only after the rift over his marriage, which his mother refused to attend, that Harry started to revise his opinion of his father. "I had taken my attitude towards him from her, an attitude of rejection and treating him rather with contempt and unpleasantness," he says. "Apart from the inherent niceness, gentleness and generosity of character, he was very patient. I think one might say he was a strong person to have coped with it all as he did." Sadly, the two had little time to make up for years of distance. Humphrey died a year after Harry's wedding.

"Unbearably strained" relations between the young couple and Marie continued. "Mary was determined to do the right thing," says Harry. Mary describes how her formidable mother-in-law, invited to visit after the birth of their first son, "bossed me around mercilessly and fed the baby things I didn't approve of". But, surprisingly, it is Mary who feels a greater sense of loss at Stopes's wrath. "I regret enormously that I wasn't tougher, wiser and older. It could have been an extremely rewarding contact. I can see all her advantages and, in funny ways, Marie and I had a lot in common - though she, I'm sure, would not agree."

Harry, imbued with the independence his mother had fostered in him from an early age, was less perturbed. "I think my hardness of character came out and carried me through." But his growing awareness of her shortcomings did nothing to dull his admiration for her vast achievements.

And while Harry has been on the receiving end of his mother's more extreme beliefs, today he is her torch-bearer, espousing her view of birth control as the fast track to social improvement. Under his influence, Marie Stopes International, the birth control charity, is working extensively in the developing world and while Marie herself opposed abortion, MSI now provides abortion and sterilisation services. The organisation has a big presence in India where Harry is sometimes required to open beautiful baby shows, which promote family planning as part of "maternal health".

The work still raises difficult ethical questions, but Harry is as dismissive of "politically correct thinking" as was his mother. "That word eugenics has been so much coloured since Hitler that one has to be careful when one uses those words and concepts," he says. But in the case of foetuses that test positive for Down's syndrome or show other deformities, he is a firm advocate of abortion: "There is no benefit to anybody, least of all to the child, to be born with Down's."

On an international level, Harry, a specialist in moral philosophy, regards MSI's work as a practical solution to a straightforward problem. "Every child born in a Third World country represents a very considerable drain on their capital resources, which means that any rise in GNP per head is being eroded by the increase in population."

Couldn't these very arguments be hijacked by Tory rightwingers in their crusade against single mothers? They could, concedes Harry, but to do so would be every bit as "bloody-minded and perverse" as his mother's objection to his own marriage.

'Marie Stopes', a 'Secret Lives' documentary, will be shown on Channel 4 on 30 November.

'Married Love' by Marie Stopes will be republished by Victor Gollancz on 7 December.