Here we go again. Following the revelation that Graham Coutts, the Brighton musician convicted of murdering special needs teacher Jane Longhurst, had been addicted to internet sites purveying images of rape, sexual asphyxiation and necrophilia, there have been outraged calls for such sites to be banned.

It's easy to see why. During the trial, for example, the prosecution said Coutts had downloaded images of necrophilia and asphyxiation and had 'acted out for real on the unfortunate Jane Longhurst the fantasies he had filed on his computer'.

The victim's mother, Liz Longhurst, who sat in court for all 13 days of the trial, said: 'I feel pressure should be brought to bear on internet service providers to close down or filter out these pornographic sites which have played such a prominent part in this trial, so that people like Jane's killer may no longer feed their sick imaginations and do harm to others'.

Easier said than done, alas - as Peter Robbins, director of the Internet Watch Foundation, pointed out on BBC Radio on Thursday. A combination of technological, jurisdictional and cultural factors rule out any comprehensive solution to the problem. In one area - images of child abuse - progress has been made. There is a Global Taskforce Against Child Pornography, an alliance of police forces who share information which sometimes leads to successful prosecutions. But that exists because there is a widespread consensus that child abuse is an evil thing.

Perhaps a similar consensus could be reached about, say, necrophilia, but I wouldn't bet on it. Beyond that, concerted action on internet pornography is a non-starter, for two reasons: first, there's a huge market/appetite for it; and second, too many powerful agencies have a vested interest in supplying it.

The moral panic over internet porn has always puzzled me. Porn merchants are not charitable organisations. It costs a lot - in server capacity, bandwidth and ISP charges - to run a pornography site, aside from the costs of originating the material. (One of the great unspoken secrets of the computing industry is how much high-end computer technology is sold to porn sites.) If there are tens (or maybe hundreds) of thousands of porn sites out there, they must be making money (and they are: the industry generates $70 billion in annual sales), so consumers must be paying.So the porn industry is meeting a need. QED.

Critics of the net often complain that it has unleashed a deluge of pornography on the world. And it is true that the web has made it easy for anyone to publish anything - no matter how repulsive. There is doubtless a great deal of 'amateur' porn on the network. But everything has to be paid for, and most of the serious (and possibly more offensive) porn cannot be accessed without giving a credit card number. It's published, in other words, for the sole purpose of making money.

And the uncomfortable truth is that it is profitable because millions of people are willing to pay for it. Which in a way explains the outrage invariably generated by the issue of internet porn. We direct our ire against technology rather than asking the really important question: why are so many people apparently interested in - or even addicted to - pornography? And what does it tell us about human nature?

The other side of the porn issue is that there are huge corporate interests involved - at every level. In the 'legitimate' world of magazine publishing, for instance, many contemporary magazines aimed at teenage girls or young adult males are just thinly concealed soft porn. The sexualisation of young girls by magazine publishers is a particularly odious modern development - yet it is done by mainstream media companies run by men in suits. The growth of Channel Five's audience was boosted in the early days by reliance on late-night soft porn. And several terrestrial channels now run sexually explicit programmes after 11pm (including a recent treatise on the care and maintenance of the dildo in lesbian subcultures).

Finally, there is the delicate issue of pornography and mobile phones. The telecoms industry recently held an intriguing conference in London on the subject of 'delivering mobile adult content responsibly'.

Let us deconstruct this. 'Adult content' means porn: mobile operators have accepted that one of the few ways of generating the revenue necessary to pay for their 3G licences is by supplying pornography. (The industry is predicting $4bn revenues annually from this.) 'Responsibly' is code for 'how to we keep children from accessing it?' I hope they succeed, but I wouldn't bet on it. Will this lead to calls for mobile phones to be banned? You only have to ask the question to realise the absurdity of it. Porn, like death and taxes, is here to stay.