John Lyttle is away. So we continue our heterosexual interlude: this week an encounter with a Rock Chick for ... Alix Sharkey

Take one white middle-class female, mature but not over-ripe. Generally speaking this is quite a robust variety, so look out for long, gangly limbs, child-bearing hips, and tapering hands that can be scrunched and flexed into many unnatural shapes. Tangled blonde curls and cool grey eyes are both good signs, especially if the former are natural and the latter are framed with sluttish black liner. Check for a curious mix of glamour and jolly hockey sticks: the overall taste should be equal parts Jerry Hall and Joyce Grenfell, squeezed into tight black jeans and a PVC jacket, with that delightful whiff of whimsical rebelliousness found only in the daughters of the Establishment.

She's toying with her Rock Chick persona. She's a five-year-old show- off in a 25-year-old body, bouncing around on the sofa in this nightclub doing this pointy, pouty thing with her nose and mouth and chin. And when she glances my way, it's not so much a look, it's the last part of an arrangement of shape and light and form that makes her real face disappear, leaving only a blank screen on to which any fantasy can be projected. It's a type of genius peculiar to bright young women. Its whole purpose is to dazzle men, and generally it works.

The trouble is that most men don't recognise this genius. They don't see this wonderful vanishing trick that creates the very space into which they project their own fantasies, their images of perfection. They don't recognise the magic involved, that it's all done with mirrors.

Instead of falling in love with this fabulous quality that has entranced them, they fall in love with a picture they saw one night, while they were entranced. And sooner or later the picture no longer corresponds to the reality of the situation.

And how do I know this? Because I read in a newspaper last week that London now has a dating agency that caters specifically for married people who want to have discrete affairs. In other words, when the fantasies of your relationship can no longer be maintained, avoid a messy and complex investigation into the cause of failure, and simply start again. I know this because recent figures show that both marriage and divorce are more popular than ever in the UK, indicating astonishing naivete coupled with incredible cynicism. I know this from age and bitter experience. Why else would a 39-year-old man be sitting in nightclub watching this process unfold once more?

Look, there she goes, she's doing it again. Or was it me?

She talks about watching the sun go down in the New Forest, and just for a second I can see the horizon she has in mind, the shafts of golden light sparkling through the trees, I can feel the sun warming my face. Talk, talk, talk. We talk about guitars, excitement, parties, beauty, Paris, afternoon tea. Talking creates space for projections, too. For example, she tells me casually that her family's former au pair still lives in Montmartre, that they're still in touch. Consider the significance of this little snippet. Imagine how easy it is to pile up a dossier of assumptions on the basis of such information.

So we leave together. We take a pirate cab. The idea is that we'll go to west London and drop her off, then I'll take the cab back to Bloomsbury. Only - amazing this - the cab driver gets it wrong and heads for my place first. By the time I realise this we're already kissing. She stops me from turning the cab around. "As long as you don't get upset because I won't shag you. But I'm not interested in one-night stands." Nor am I, says somebody, in a faraway voice. Her head is resting on my chest and her eyes are closed. I realise what I have just said and, unbelievably, that we both know I mean it.

We sleep together, nothing more. In the morning I cook mushrooms, heat the beans, put the toast on, and drag her out of bed to cook the omelette. She wears my old Levi's with holes in both knees and a purple Body Shop sweatshirt. She has unwittingly put it on back to front, bless her. What a groovy Rock Chick. What a sweet girl.

Over breakfast we discuss our families and her ex-boyfriend, a model. During the two years they lived together she was obliged to give up her friends, her band, and going out. The ended up having arguments in the morning over who should use the hairdryer first.

"So don't ever mention the `R' word to me," she says.

I don't know what that means, I tell her.

This strikes me as perfect. So perfect, in fact, that I realise I must have imagined it. Arguments over the hairdryer. Dating agencies that specialise in adultery. I guess it always starts a little more romantically than that. With a sunset in the New Forest, perhaps.