My partner and I have been very much in love for about seven years. Early on, she told me about a man she had met briefly, years earlier, to whom she felt a sexual attraction unlike anything she had experienced before. Nothing happened, so it stayed an unexplored fantasy.

A year ago they met again by chance, and the chemistry was (she explained) instantly electric. We had a frank discussion about her desire for him, which resulted in her taking him to bed one afternoon for what she described as the best orgasm she had ever experienced. This was followed by weekly lunchtime sex (at my apartment in London).

Soon afterwards, my partner and I decided to live together, and her lover began to visit our place once or twice a week. The challenge, for me, was to recognise that the chemistry she had with him was no threat to her love for me. I could not ask her to forgo such enjoyment (he is 20 years younger than me, 15 years younger than her, so there is a huge difference in the pleasure she gets from each of us). I recognised that the problem was mine, not hers.

We interact comfortably and they feel unselfconscious about showing their mutual attraction in my presence. I enjoy seeing their foreplay or hearing them having noisy sex in the spare room – not as a matter of eroticism, simply two people having a good time.

She would not want me to have the same, and so I remain happily monogamous. Through honesty, openness and trust, we have found the formula for a perfect model relationship.

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