Nicholas Comfort, a mild-mannered grandfather in his mid-70s, is the very soul of discretion.
A heritage steam railway enthusiast — he volunteers with the Bluebell Line in West Sussex — cricket aficionado and elder of his local free church in South-East London, he went to public school, progressed seamlessly and with distinction to Cambridge and has enjoyed a career as a journalist, writer and political adviser.
He dresses with conservatism, his hair is neatly barbered and his demeanour youthful.
Nicholas Comfort, the only child of the late Dr Alex Comfort who authored The Joy of Sex, and wife Jeanette
The Joy of Sex, launched in 1972, has sold more than 12 million copies and has been translated into 30 languages
You'd be forgiven for thinking, with his irreproachable credentials and impeccably respectable hobbies, that he was the product of an entirely conventional middle-class upbringing.
But how deceptive appearances are! For Nick is the only child of the late Dr Alex Comfort; author of The Joy Of Sex, the ground-breaking erotic manual with which his name became synonymous. And this year marks 50 years since its publication.
Not that his son, it becomes readily apparent, shares his father's ease with eroticism. Bring up the subject of sex and he deflects it deftly.
Was he a proponent of free love as a young man in the Swinging Sixties?
'I was heavily into music. I went to a Pink Floyd all-nighter and saw the Stones at Hyde Park,' he discloses as if this is the most shameful act of depravity.
'But I'm not going to do a Nick Clegg and tell you how many sexual partners I've had,' he adds, somewhat primly. The former Lib Dem leader infamously told an interviewer he had slept with 'no more than 30 women'.
'Let's just say I came away from university a great deal more experienced than when I went to it,' Nick adds in his genial, family-friendly way.
As father-son dynamics go, it's hard to imagine one more polar opposite — although there are hints of mischief beneath Nick's outward conservatism.
He married his third wife, former bunny girl Jeanette, with whom he now lives in South London, after meeting her on a train. Even so, it is much easier to envisage Nick writing a political treatise than a 'how to' book on his bedroom antics.
The Joy Of Sex, with its mix of humour and frankness, piloted us through the mechanics of imaginative love-making, its illustrations showing a couple in the throes of ecstasy, adventurous experimentation and post-coital bliss.
It was divided into Starters, Main Courses and Sauces and Pickles, it was loosely modelled on the Joy Of Cooking; and just as the culinary manual wanted to induce proficiency in the kitchen, Dr Comfort's book aimed to make its readers capable of Cordon Bleu sex.
It has sold more than 12 million copies, been translated into 30 languages and extraordinarily when it was launched in 1972, on an eager public emerging from the constraints of post-War prudishness, it was considered wildly outre.
Hard to believe now, in a world of untrammelled internet porn, that the summer before 'Joy' (Nick's sweet abbreviation) was published, Britain had been enthralled by a high court trial, in which the editors of the satirical magazine Oz were found guilty of obscenity for publishing a sexualised parody of Rupert Bear.
Half a century ago, The Joy Of Sex caused a similar public furore, yet today its message seems affectingly moral.
'You don't get high-quality sex without love and feedback,' opined its author, who actually enjoyed an extraordinarily racy sex life himself.
While married to Ruth, Nick's mum, whom he met at Cambridge University, Alex Comfort enjoyed an affair with Ruth's best friend Jane Henderson, whom he went on to marry.
Dr Comfort and his second wife then emigrated to America, where they became enthusiastic adherents of free love, joining the Sandstone Retreat, a clothing-optional, open-sexuality resort for swingers in the Santa Monica Mountains.
Poor old Nick. We can only imagine his chagrin.
'My mother was not the most tactile of people and Jane, presumably, was,' Nick says with wonderful understatement.
'Dad was dividing his time between mum and Jane and this arrangement almost became a fixed state.
'But then his research work (into gerontology, the study of ageing) at University College London came to an end, he went to America and 'Joy' came out.
'Dad told me later there were quite a few gerontologists at Sandstone. He said all of them had a chronic fear of their own mortality which made them want to enjoy life more.'
His mother must have been hurt by the betrayal, surely?
'It would have been impossible for her not to have been,' concedes Nick.
'And in an ideal world Dad would have liked not to have made a formal split. But Jane was the needier of the two. My mother was better able to function on her own.
'In fact I think she found Dad's intellectual explosiveness and high-octane mind difficult to live with and, although she was hurt, in some way not having him around gave her a chance to recover.
'Dad was still very fond of Mum and he felt desperately conflicted.'
While Alex and Jane threw themselves into their libertine lifestyle, they were mercifully reticent about the goings on in California whenever they met Nick.
'I think they believed that what went on in Sandstone, stayed in Sandstone,' he says.
'Ironically, Dad was a fairly private man. He didn't talk about his own sex life. Very few people do. He talked in generalities rather than in the particular.'
But even before his father went on his odyssey of sexual experimentation, and published The Joy Of Sex — which was based on his erotic adventures with Jane — he was making radical pronouncements.
Nick was 17 and a boarder at Highgate School in North London when Dr Comfort announced, daringly, on a radio broadcast — this was 1963 — that no responsible teenage boy should go to a party without a condom in his pocket.
'I wasn't embarrassed until people started teasing me about it,' says Nick. 'And they did.'
If he availed himself of his father's advice at that stage, it was redundant.
