Is it ok to have sexual fantasies about someone else when you're in a serious relationship?

Mmmmm. Sexual fantasies. The most portable sex aid – and, arguably, the one which causes the most distress.

Fantasy is just that – a pleasant daydream with erotic potential. Nonetheless, many of us feel that it’s a betrayal of real life relationships, especially if it involves sex with someone other than our partner – and it often does for 98% of men and 80% of women, according to the Journal of Sex Research.

Recent research confirms that sex with someone known to them who is not their partner is in the top ten fantasies for both men and women. The survey also revealed that the majority of women in the sample kept their crushes a secret from their partner.

Since it’s so common, it might seem like a good idea to tell your partner who you’ve been fantasising about, but unsurprisingly this can backfire. Partners sometimes find it difficult to grasp that fantasies aren’t necessarily something we’d actually like to do. If you tell your partner that you fantasise about a celebrity, your boss, your ex, or, worse still, your partner’s best friend, the risk is that they start feeling threatened and jealous. The next thing you know, you’ve been accused of having an affair when all you’ve actually done is had some thoroughly entertaining thoughts about your colleague involving an aubergine and a rolled up copy of The Independent.

Often, good sexual etiquette can mean adopting a ‘don’t ask-don’t tell’ policy and keeping our fantasies to ourselves. Indeed, the more unusual fantasies are – and some of the best are deliciously bizarre – the more they can lose their usefulness when shared. The thing is, reality can sometimes spoil the story and encroach on the private world you’ve created to lose yourself in. If you decide to try the fantasy out, it can also be hard to reproduce as you’ll inevitably be affected by the surroundings, your partner’s ideas about how it should go and your own feelings of having ‘lost it’ once it’s out there.

Co-created fantasy, on the other hand, can be a lot of fun. If you want to act it out, it’s important to discuss the story, protocol and safe words or behaviours which mean ‘stop’ beforehand.

Solo fantasy may be less effort, and research confirms that fantasising can hugely enhance lovemaking and form a healthy and desirable part of couple sex. The focus of the fantasy is usually more on ourselves than on who we’re having sex with. During fantasy you can be whoever you like – as sexy, attractive, powerful, submissive, skilled or innocent as the mood takes you. The imagery and the way fantasy helps to block out the world and focus on the pleasure can be what you need to lift you away from the stresses of everyday life.

Some people may not reveal fantasies which they believe aren’t politically correct or which involve their own subjugation, but for some people these are incredibly arousing and in no way suggest they want to or will act them out in real life.

Whether they’re aids to masturbation or used in partnered sex, some of us worry that our fantasies are odd or perverted. But the fact is that many of us use anything from the down to earth to the wonderfully unusual as a means for getting what we want from sex.

Way back in the 1960s, the feminist journalist Nancy Friday began researching women’s sexual fantasies, subsequently producing graphic compilations of the fantasies of both men and women. Her first book the blood pressure-raising My Secret Garden - proved once and for all that women do think about sex an awful lot. The broad range of fantasies included a woman visualising her lover having sex with her friend while she’s busy putting away the groceries.

The variety and creativity which Friday’s work revealed seems to be just as alive today. Visitors to the Institute of Sexology exhibition at the Wellcome Collection in London have been answering a series of questions about their sex lives as part of an installation, Would You Mind?, devised by theatre director and author, Neil Bartlett. So far, more than 10,000 people have taken part, providing evidence as to the diversity of our sexual fantasising and richness of our imaginations.

If you still don’t like the idea of what may be happening in your own partner’s head, look at it this way: you’re going to be more turned on and feel more affirmed if your partner’s pleasure is obvious, regardless of what helps to get them there. Or just close your eyes and use your imagination.

Cate Campbell is a Relate counsellor and psychosexual therapist

The Relate Guide to Sex and Intimacy by Cate Campbell is published by Vermilion on 6 August 2015, priced £9.99. For more information visit www.relate.org.uk/sexguide