Jemima Lewis: The difference between sex and pornography

Long before I tried it, I knew that sex could be funny, miserable and embarrassing as well as passionate

Named after its first modern collector, the American tycoon Ned Warren (who was also responsible for commissioning Rodin's sculpture The Kiss) it was hitherto considered too obscene to go on public display. But the British Museum bought it for a whopping £1.8m in 2000, so it can hardly be blamed for wanting its money's worth.

The cup has gone on show with a parental warning sign, of the type more often found on Eminem CDs: "Please note that the display contains sexually explicit images." This is, perhaps, intended to entice as much as inform. In recent years, museums all over the world have been dusting off their ancient erotica in an attempt to lure in visitors who might not normally appreciate Greco-Roman artefacts. Even the famous Secret Cabinet at Naples archaeological museum - once accessible only to educated males of "respected morals" - has flung open its doors to the lumpen proletariat.

Like all venerable institutions, museums are especially anxious to attract the young - for whom sex has traditionally been the most effective bait. But, sadly, it seems this may no longer be the case. At the launch of The Warren Cup: Sex and Society in Ancient Rome, the visitors were predominantly middle-aged women in large glasses, peering at the scenes of sodomy with scholarly interest. Young people aren't impressed by classical porn any more: they are up to their ears in the real thing.

A few weeks ago, my boyfriend found himself sitting behind two 11-year-old boys on the bus. It gradually became clear from their loud, boastful conversation that they were looking at porn on their mobile phones. My boyfriend - not a man who is easily shocked - peered over their shoulders. "It was horrible," he reported back, ashen-faced. "Way too nasty for my taste."

He, like me, grew up in the pre-internet age, when sex was a mystery that could only be unravelled slowly. When I was 11, I thought intercourse was an accident that happened when a sleeping husband rolled over in bed and landed on his wife. It was painful and embarrassing, but quickly forgiven because you ended up with a baby.

This much I learned from a boy at school called Phil: the rest I had to scavenge piecemeal from novels, paintings, magazines, saucy postcards - whatever artefacts of grown-up culture I could lay my hands on.

I didn't want to be told the facts of life straight out, least of all by my parents. (My mother did try once, but I ran around the room with my fingers in my ears, making Red Indian whooping noises until she gave up.) Instead, I stole a selection of romantic novels from her bookcase and read them, my heart booming with excitement, under my bed.

There, amid the dust balls and discarded socks, I discovered that sex - or at least kissing, and being clasped by men with powerful hands - was something people actually enjoyed doing, because of this condition called love.

From a feminist perspective, there may have been flaws in the Mills and Boon method of sex education, yet I can't help thinking it is greatly preferable to the one on offer to modern children.

A survey two years ago found that 57 per cent of British children aged nine to 19 have looked at pornography online. That figure is likely to have risen since: a more recent American study put the figure closer to 70 per cent. It also found that, out of the entire global population, the largest group of viewers of internet pornography was children aged 12 to 17.

These days, any curious tot can go online and shed her innocence in a matter of minutes. But what she learns from the internet - graphic though it may be - is not the truth. It is a two-dimensional, cartoon approximation of sex, resolutely lacking in feeling. Previous generations, who had to build up their sexual knowledge in layers - a bit of Donald McGill here, some Henry Miller there - at least got an impression of the multiplicity of emotions involved.

Long before I ever tried it for myself, I knew that sex could be funny, miserable and embarrassing as well as passionate. You would never learn that from the glassy-eyed mannequins on the net.

The radical feminist Andrea Dworkin once argued that "erotica is simply high-class pornography; better produced, better conceived, better executed, better packaged, designed for a better class of consumer".

But she missed an important distinction. Erotica - at least the good stuff, like the Warren Cup - is full of feeling. The very fact that someone has laboured for days with a tiny hammer and a piece of silver to depict two men making love makes it moving. It's an education in romantic feeling - and that's something that every child needs.