It was a Friday night a few months ago. Rob was standing on my doorstep, ashen and trembling. He still couldn't speak even as he sipped at a mug of tea after my flatmate and I had ushered him into our front room. We could not guess what had happened, but a feeling of dread was fast forming in our minds; we could only assume that something terrible must have happened to Rob's fiancee, Karen. Gradually, his powers of speech returned and the story emerged. Something had happened, but that something was terrible to Rob himself, not Karen.

Now, you have to understand that Rob and Karen were the most balanced, wholesome couple I knew. They had recently moved out of their flat while it was being redecorated, each returning to their respective family homes for a couple of weeks. That Friday, she had left her keys at his place by accident. He was passing by her parents' place later that night, so he stopped off and rang the doorbell.

No answer. So he let himself in to leave the keys, with a note, on the kitchen table for her mum and dad to return to her. Only, the house wasn't empty - he heard some movement in the front room. In an instant, he blundered in on Karen - the woman he was due to marry - having sex. With her dad.

Not her stepfather. Not her adopted father. Her actual, biological dad. She was 22 years old. There was clearly no coercion taking place.

Three weeks passed. Rob had called off the wedding - obviously - and was trying to put his life back together. One morning, he got a call from Karen, asking if they could meet up to divide their mutual belongings, the accumulation of over three years' cohabitation. He agreed.

Predictably, when they met, an argument began. "I don't know why you think it's so odd!" she screamed. "I know lots of people who do this." That stopped Rob in his tracks. "Who?"

And it began to spill out: that she had made contact with lots of people over the internet (and consequently in person): boys, girls, fathers, mothers, who are sleeping with their kin. The internet is value-free: it doesn't care or know whether you are selling a secondhand car or buying arms. If you want to get in touch with someone, it makes no moral distinction between anti-globalisation protesters and convicted paedophiles. So now there are chatrooms and websites that are de facto support groups for people engaged in incest. And what they want is to normalise what we have long considered to be profoundly abnormal.

It was on this basis that Karen said Rob was "overreacting" - she had insinuated herself into an online "community" of people who reassured themselves that they were not freaks. Rob and I spent a few nights gawping at the disguised but fairly developed pro-incest (or, to be more accurate, pro-tolerating incest) areas of the net in an attempt to understand Karen. The exponents of incest that we talked to in cyberspace were very keen to draw a distinction between "consensual incest" on the one hand and abuse, rape and paedophilia on the other. Consensual incest, we were told by "JimJim2" from Ontario, is "when two adults who just happen to be related get it on. You can't help who you fall in love with, it just happens. I fell in love with my sister and I'm not ashamed ... I only feel sorry for my mom and dad, I wish they could be happy for us. We love each other. It's nothing like some old man who tries to fuck his three-year-old, that's evil and disgusting ... Of course we're consenting, that's the most important thing. We're not fucking perverts. What we have is the most beautiful thing in the world."

Voices in Action, a US support group for victims of incest, vehemently rejects these arguments: "These teens have been brainwashed into believing this behaviour is natural; it is not ... Sexual abuse is learned behaviour." But some political thinkers are prepared to support the distinction between abuse and consenting relationships. Dr Sean Gabb, a leading member of the Libertarian Alliance, a radical British thinktank, argues that "consenting incestuous behaviour is no business of the state. It is up to individuals to make their own decisions." He has drawn attention to the "unjust" 1909 case of R v Ball, where a seemingly happy brother-sister couple who had been living as man and wife were "outed" and thrown into prison. He describes them as "harmless and respectable".

Few other public figures are prepared to tread into this ethical minefield. One of the few who was brave enough to talk on the record is Brett Kahr, senior lecturer in psychotherapy at Regent's College, London. He stresses that there is no proper research into this phenomenon, and wonders, "Who are we to say that Joe Bloggs and his sister Jane Bloggs aren't having a perfectly good relationship and we're all missing out?"

But he is also quick to qualify this. "In over 100 years' worth of case studies I've looked at, I have never seen a single case of incest that has ended happily. I don't know a single experience where an incestuous relationship has been positive." He admits, however, that - by their very nature - psychiatrists don't attract happy, functional people. For example, the couple in the case cited by Dr Gabb, whose court transcripts suggest they had a perfectly happy life, would never have come to the attention of a mental health professional.

Kahr, drawing on his experience as a practising psychotherapist, raises some pertinent questions about any incidences of seemingly "consensual" incest. "Even if, as in your friend Karen's case, she did, as she claims, initiate sexual contact with her father, what was lacking in her relationship with him so that sexual behaviour seemed the only way to bridge it? She may have been behaving sexually because she had failed to attract attention in any other way."

There is a surprisingly wide range of literature concerning incest for us to draw on when we try to understand the mindset of the participants. Consensual incest has been portrayed sympathetically in popular fiction for centuries, from John Ford's masterful 17th-century play 'Tis Pity She's a Whore to Ian McEwan's novel The Cement Garden and Steven Poliakoff's film Close My Eyes. The writer Kathryn Harrison caused a sensation in 1997 when she published The Kiss, a memoir of an affair with her father, and even in the popular medium of TV, from Jerry Springer (who has featured incestuous sisters on his show) to Brookside (which featured a love affair between siblings Nat and Georgia), incest has been depicted in a not unsympathetic, if somewhat salacious, manner.

So why is your stomach still churning as you read this? What is it about incest that makes it universally abhorred? The most obvious answer is the risk of producing severely deformed children. King Hatchepsut, an Egyptian pharaoh who was the product of an incestuous union between brother and sister, is considered by many Egyptologists to have suffered from birth defects. In Michigan in the mid-1990s, the state laws had to be reformulated to forbid "consensual incest" after two high-profile scandals. In both cases, the offspring of father-daughter relations had severe birth defects and several of the resulting babies died. All existing studies of inbred populations show that incest increases the rate of appearance of negative recessive genes.

We should, however, be wary of damning incest on these grounds alone. To prohibit two people from having sex because their offspring may be "defective" or "inferior" is to adopt the standpoint of a eugenicist. Indeed, Dr Sean Gabb has clearly shown that the impetus behind the 1908 Punishment of Incest Act was just that: the proponents of the act were exactly the same figures who advocated the "sterilisation" of the "feeble-minded". If we prohibit incest on the grounds that it risks producing "defective" children, we must also prohibit reproduction by haemophiliacs and the carriers of a host of other "defects".

In any case, we must acknowledge that, with the rise of contraception, we have succeeded in separating sex from reproduction. Another unashamed participant in incest discovered in a chatroom, "daddysgirl", insisted: "We would never have a baby, it would be all screwed up and wrong. I use the coil." So has a window opened for "safe" incest? And if so, is our visceral disgust just a remnant from a vanishing age?

· Some names have been changed.