A THREE-IN-A-BED session can sound like a lot of harmless fun but I know this issue causes a great deal of unhappiness and conflict between many couples.

The most common situation I hear about from readers seems to be that it`s the man who first raises the idea of the couple taking another man or woman into their bed.

He finds thinking about it very arousing - it`s almost like an extension of reading a sexy magazine or watching a video, a sexual fantasy. He may have been finding his ordinary married or regular sex life a bit dull, so goes on and on at his partner not to be prudish.

She may or may not agree to give it a try but if you are caught in this dilemma, whatever stage you have reached in this, let me suggest you stop right now.

Perhaps there are a few couples who happily manage three-in-a-bed sessions on a regular basis and never let it affect a close and loving relationship, but they are so rare I have never come across them. Yet week after week I hear from couples who have experimented and been made very miserable.

The trouble is that real people don`t behave like the fantasies and usually have very muddled feelings.

A husband who has pushed his wife into making love with another man may then reproach her for being unfaithful to him - "You should have kept saying no," he moans.

He may say he now feels he can never trust her again and leave - what a way to discover that what was arousing him wasn`t really the idea of "swinging" but old-fashioned sexual jealousy.

Or the woman may decide she`s fallen in love with the other man. We all have some problems in long-term relationships. It`s all too easy to fall for the idea that someone you`re sharing different and exciting sex with would actually be a better life partner, is a lovelier person.

Sometimes it`s true. More often they`re just a different person - and it`s always worth wondering what sort of person they really are to get dragged into all this in the first place.

If it`s another woman you`ve invited into your bed, then all the same problems can arise the other way round. A wife may find she`s tormented by images of her husband with the other woman in their bed.

She may have agreed to go along with the idea of a three-in-a-bed session, may have thought it sounded like fun, but afterwards discovers it`s ruined the happy sex life she and her husband used to share.

That can place a terrible strain on their relationship even if he never sees the other woman again.

Even worse, a wife may find she`s been sidelined and her husband is clearly keener to make love to the other woman than to her. She may worry - and perhaps have good reason to worry - that they are having an affair.

Long-term relationships always have their niggles and shared responsibilities. A different partner can make you feel better not because they are a better lover or person, but just because they aren`t directly connected with your day-to-day life. It`s escapism but, unfortunately, someone always winds up hurt.
sections: Culture