There are some things you just can't say to a girlfriend, however close you are to her. One is, "Why are your kids so awful?" And another is, "Are you good in beds" Your friend may describe her man as a "stud with a tremendous body" but don't expect her to tell you how he uses it - or how she uses hers. The specifics of who did what and to whom, are just not something nice girls talking about.

That's the trouble with sex. It has no rules, no guidelines, just pleasures and anxieties. Being good in bed is one of those things you worry about. How do you know what other couples do: What if every woman in the world is better than you are: How can we be sure that we know all there is to know?

For answers, we usually look to the movies. But movies are so imprecise when it comes to something most of its practise twice a week (I say "twice a week", but I'm just quoting something I read - frequency is one more thing women are vague about). How is it that in movie, everything turns lovers on? He only has to stroke her neck and her back's arching and her eyes are rolling back in her head. And when it comes to penetration, there are no wriggling, contortions, repositioning or cries of "Not there!" or "Ow! I'm not ready". Naturally they climax together.

And when it comes to oral sex, a woman's always ready. She only has to dive down into the sheets and suddenly his head's thrown back, lie's thrusting and writhing, and groaning so loud you think she must be amputating his leg. You long to nudge your friend and say, "What is she doing to him?" But you don't because maybe your friend knows what this woman is doing. This just confirms what you always suspected. That everyone is better in bed than you are.

If movies give you an inferiority complex, then self-help books make it worse. The Joy of Sex, for example, describes how to contract the vaginal and pelvic muscles to give a man extraordinary pleasure. These Kegel exercises are said to him a cinch for any woman to learn. So why can't I do them? If you ask me, how-to books are sometimes just a vehicle for the writers to demonstrate how much better they are at it than we are. And they must be better because when they try a new position for sex, they don't fall over/bang their heads/slide off the bed/lose their temper and go to sleep in a huff. Maybe if we had entire weekends to devote to sex instead of entire minutes, we'd be sex experts too.

If books hadn't told you. I certainly wouldn't have guessed that the pelvic muscles were a powerful sexual weapon. Or what about toe sucking: Five years ago I'd never heard of it, but suddenly- everyone from Sarah Ferguson's beau to the ex-publicist of Marla Maples, ex-wife of Donald Trump seems to be doing it. I feel such a fool. I've bought shoes, completely unaware that the sales assistant might he inflamed with desire by my naked instep. Did no one say anything because they didn't know about toe sucking either? It's disconcerting when you discover there are things you've never even considered. Mutual masturbation, edible panties, leather and bondage - should you, shouldn't you

Take whipped cream. Comedians are always making jokes about whipped cream and everyone laughs. But is it funny because everyone uses it - or are they just laughing at the perverts who do? Will trying it put you into the "good in bed" class or the "ugh - you didn't!" category. And when is pleasure perversion anyway? Is it abnormal to want to dress up in high heels and suspenders - or only when he does its

If we could just discuss it all, we wouldn't get so jittery. But maybe women are born shy about these things. Young boys have enough euphemisms for the penis to fill a small dictionary; while girls may hardly know any for vagina - and if they do they still don't say them.

As women get older: they may become more sophisticated and experienced but sex talk, even with their closest friends, still tends to revolve around generalities and innuendo. My friend Paula once chanced upon her friend's open drawer in her home. "She slammed it shut straight away, but not before I'd seen that there was a pair of handcuffs in it," recalls Paula. ''She knew I'd seen them but we still acted like I hadn't. Until then I thought there was nothing I couldn't discuss with her but I couldn't have asked her about that."

The fact that we can't discuss it just makes us all the more curious to know about other people. You look at women on the bus and you think. "Would she do what I was doing last night?" Or you meet your friend and her husband, whom maybe you don't like, and you think - would she have oral sex with him Or you look at the quiet one in the office - who only smiles shyly at all the banter and innuendo - and you think. "What does she do in beds?"

A few years ago, a friend from the local church gave me a hint, more or less. She had bought a black bustier with little red bows. It was pinched in at the waist, and thrust out at the bust and she also had seamed stockings to wear with the black suspenders. I wanted to die. Because it had never occurred to me that real women - except those posing in men's sex magazines - would ever wear that sort of thing. But if even a pillar of the church community wore them, then everyone must be doing the same and I must be really unadventurous and unimaginative. Having insight into other women's sex lives isn't always encouraging.

I once showed a friend a passage from a novel which described what I thought was a weird sexual perversion. My friend blush a little and looked affronted. "I can't see what's so awful," she said. "I expect a lot of people think it's very nice" - clearly implying that she was one of them. No doubt it's the same risk of embarrassment which prevents men from discussing the realities of sex with their friends. As a friend, you can never know for sure how good you are. Discussing it with friends could mean you lose face."

But oddly enough, men and women don't mind talking about it to each other. It seems to release both sexes from the fear of judgment we might feel from our own sex. Even if we know the moves, can we be sure we're doing them right? Your partner might tell you this of an ex-girlfriend: She was no good in bed, but you are." But you don't think you're particularly adventurous. So just what are the criteria

Technique isn't important at all. "As far as I'm concerned, a woman is good in bed if you've got a good relationship and she's telling you honestly how to give her what she wants. It's about communication." It isn't a case of the tricks she knows but how she responds. "If she's excited, he's excited." So at least part of being good in bed is making him feel that he is. And this, I suppose, is why being "good in bed" implies having orgasms - because if you don't have one, he thinks he hasn't done his stuff.

Even so, women have to accept that some women genuinely do have great technique. The most exciting woman I ever dated was very inventive and uninhibited about taking control. The element of surprise is very provocative. It's pretty sobering to realise that there are such great exponents of the art around - especially so if you're naturally self-conscious and your idea of surprising him is to say "yes" to sex when you normally say no.

Perhaps we should accept that there are some women gee just can't compete with. But it's a mistake to assume that all women want to compete. I know someone who never undresses in front of her husband and would be shocked if he saw her naked. Another woman is almost equally reserved. "I think my husband would like to be more experimental but I certainly wouldn't do anything kinky, especially not oral sex. If you start getting very active, then you're just making love the way a man would, and it's not feminine." My friend Fiona claims to be equally inactive in bed these days, but for a different reason. "When you've been together a while, you don't have to impress him. I feel now that if I'm not in the mood then I don't have to do it. It's been a very liberating discovery."

Maybe it's not been quite so liberating for her husband. In my experience, you should pull out all the stops occasionally to make up for the times you just lie there thinking about defrosting the refrigerator. The truth is, you don't have to be the best lover a man ever had - or even if you think you are, you don't have to hit the heights every time. After all, marriages aren't perfect in other respects: women don't assume women cook the best food he ever tasted, that women bring up the kids better than anyone else could, or that were the most beautiful woman he ever set eves on. So why worry if women are not his best ever when it comes to sex?

I asked Jane recently married to a man who freely admits to a colourful past sex life how she would feel if she found out that a previous girlfriend had been a better lover. "Other women may have been sex bombs in bed but what he did before he met me is done and finished with. And anyway sex isn't a competitive sport." And as she points out: "It's me he loves. I'm the one he married.'