I'm 28 and my girlfriend Sarah is 26. We've been together for three years. On average, we make love every 10 to 14 days, though she claims it's at least weekly. For a couple of years I have tried to initiate it three to four times a week and have been refused with, 'I don't feel like it'. She says it's nothing to do with me or the relationship, and that in the early days she had more sex to please me and has now settled into her natural pattern. She was suspicious that all I wanted from the relationship was sex and I have to prove to her that this is not the case. I feel unmanly and it hurts to be like a weak man. Lying next to her every night and not being able to touch her is unbearable. I would like us to get some help, but she will not admit there is a problem.

It's important to realise what is 'normal' when it comes to the frequency of having sex to help you put your experience in context. In the definitive sexual survey of 19,000 Britons, the average cohabiting couple in your age group (25-34) do it six times a month. That's twice as much as you by your reckoning and one-third more than Sarah's (once-weekly) estimate.

However, you have to take into account that this study will have included many couples who have not been together for long, and that has a big effect on frequency; another large survey found that the average rate of sex had dropped to twice a month in relationships after the third year, regardless of marital status. Even the young went off sex if they stayed together for that long.

Of course, statistics just deal with the average, but part of your frustration probably does derive from notions of what is 'normal', so it's important to grasp that, for a couple who have been together for three years, having it as often or rarely (depending on how you look at it) as you do is probably very common.

Apart from anything else, this should be solace to your wounded masculinity. It may have nothing to do with you that she is not keener - she might well feel like this after three years even if she was with Liam Gallagher.

Nonetheless, it's understandable that you seek a fuller explanation than 'I don't feel like it', so let's consider some possibilities. The strongest clue comes from her fear that you only wanted her for the sex. It's possible she has never seen sex as an important reason for getting into relationships and that it is the security of being close to someone that she really prizes. If so, she may have felt abandoned in her early years. If this is so, then she would benefit from some therapy, because it's a problem that is eminently treatable and, as a result, she might well get much keener on sex. Except that, if this happened, you might not be the only person she wants to do it with. If she has never really experimented much or had many partners, having become more secure she might start making up for lost time. So while it's frustrating that she does not want more sex, at least it means you have got a faithful partner, for now.

Alternatively, it could be that she is just an old-fashioned girl. In the past, the saying 'Men use love to get sex, women use sex to get love' was frequently true. This is less true of women in their teens and early twenties today. Maybe Sarah was just brought up traditionally.

Anyway, it's important for you to get your head together. That means looking closely at why the lack of sex makes you feel so unmanly.

Like most of us, you probably have some fears about your attractiveness and potency, that you are not sexy enough and are not pressing the right buttons. I very much doubt that either of these are the case in reality, it's just the way you feel about yourself deep down. But there is no need at all, as Sarah says, to take her lukewarmness personally. You need to rethink what you really want from the relationship: if you are expecting wild and frenzied passion after three years, it's rare, and she's not likely to provide it.

Perhaps the reality is that you are not yet ready to settle down and maybe it's run its course. You are still young enough to be fairly highly sexed (on average, men need it a lot less in their forties), so perhaps you need to get out there again as a single man.

Alternatively, you may feel that this is the woman for you, sex three times a month or not. In that case, it might well be worth, as you suggest, both of you going to a Relate counsellor and looking at it from both sides.

If you do, you will have to face the fact that you too are bringing problems to this situation, and putting it like that might well sell it to her as an idea.

· Oliver James' book They F*** You Up: How to Survive Family Life is out in paperback. Mariella Frostrup returns in two weeks.