What sort of marriage lets a husband take on lovers?

For most brides-to-be, this would be a deal breaker, but Sally saw things differently. "She had come out of a very deceiving relationship," says Russell. "She doesn't like deceptive people. When I met her, I had several lady friends and she accepted that about me.

"We knew we loved each other and we wanted to have a family together. It just felt right."

When the York couple married ten years ago, they set some ground rules.

"They were not to bring anyone to the house, be honest if asked a direct question and not to involve the kids," says Russell, who now has two children with Sally.

A decade on, the couple have a healthy marriage. "We have sex almost every day," says Russell.

He insists his relationship with Sally and the kids is the main priority in his life, but he still needs and enjoys the freedom to have sex with other women.

"I've always been a bit of a flirt, that's one of the things my wife likes about me: that I've got a bit of a sparkle in my eye," says Russell.

In the early years of his marriage, Russell had more girlfriends. Today, it is rare that he sees other women, but he says it is important he has the right to be with other women, should the opportunity arise.

He would not call his actions infidelity and argues faithfulness and monogamy are different concepts.

"By faithful I mean I am going to live with my wife for the rest of my life, and bring up our children together and make sure I reward her for all the love she gives me," he says.

Sally, Russell says, chooses not to "take up her right" to have other partners. Russell admits it would be hard to visualise her with another man. "If my wife fell for a nice fella, I would try to feel frubbliness' for her, but the truth is I don't know."

"Frubbliness" is a term coined by the "polyamory" community - people who practise having relationships with more than one partner. It means to experience joy when you see your partner loving someone else. Another word for the same emotion is "compersion".

Such is the taboo nature of open relationships that Russell and Sally have asked us to conceal their identities.

Author Anna Sharman shines a light into this secret world in her new book, Open Fidelity, An A-Z Guide (Purple Sofa Publications, £4).

The slim guide gives a brief overview of plural relationships, pinpointing potential problems, from jealousy and time management to telling your children, as well as examining the plus points, such as help with childcare with all those extra adults around. Russell was one of her interviewees.

Anna says: "I am not advocating infidelity. That means breaking your promises to your partner. Secret affairs are destructive. However, a non-secret affair, having other lovers with the full knowledge and consent of your partner, is another thing altogether."

Relate, which offers couples therapy, said people considering an open relationship might benefit from counselling.

A spokesperson said: "Unfortun-ately, many committed relationships do not have an equal power balance, and the introduction of extra partners can become coercive, at a very high emotional cost. The implications are serious for the health of the other partner - self-esteem may be undermined, compounded by jealousy and acute insecurity.

"For most people, a monogamous relationship offers security within the boundaries of exclusivity. Once it is upset by the introduction of new partners that security can be lost. For a couple considering this step it is always crucial to look at the primary relationship and be honest about what it lacks and what it needs.

"There is also the issue of what if' - what happens if I like my new partner more than my current partner and decide to move in with them? What about the wider implications, particularly if there are children involved?"

However, Russell and Anna believe open fidelity works, the key is negotiation and honesty.

Anna says: "Open fidelity means discussing and negotiating any outside interests before you have sex with anyone else."

Russell says an earlier relationship broke down because of his secret affairs, and he much prefers being honest about his extra-marital activities .

He says: "My wife doesn't want to know the details but if she asks me a direct question I will answer honestly, which feels weird after so many years of being deceitful."

The issue of plural relationships has been highlighted in the TV series Big Love, where actor Bill Paxton plays a successful businessman with three wives. Besides the secrecy, time management proved to be a huge stress factor in his life.

Russell says it is crucial that anyone practising open fidelity makes their primary relationship their priority.

"It's natural and decided if you have children to focus on them," he says. "If you spend time nurturing your relationship at home, you can go out once or twice a year to a swinging club or visit a prostitute or girlfriend and it's OK; it's like having a holiday."

He says he has spoken out to encourage people to be more honest about their needs and relationships. He says: "I'm not doing this to titillate, but because society needs to develop. If people were more honest we'd have a healthier society."

And he would like people to think twice before judging him. "I'm an ordinary guy who can't say no to sex if it's offered, but instead of having to hide this from my wife I can be open about it and she accepts it as normal."

From Branson to Einstein...

SECRECY surrounds the issue of open marriage so it is difficult to assess its prevalence. However, researchers estimate that between two and six per cent of married people are involved in open marriages.

Tycoon Richard Branson has spoken about how his first marriage hit the rocks because of open fidelity'. In his autobiography he tells how he lost his first wife, Kirsten, after a wife-swapping session with a rock star.

Last year, Diane Melly's Take A Girl Like Me gave an account of her life with jazz star husband George Melly and their open marriage, which proved more successful for him than her.

We might associate the notion of open marriage with the Sixties and Seventies, with the advent of sexual liberation and swinging parties, but the concept pre-dates that.

Among historical figures reputed to have had open marriages are Liberal Prime Minister David Lloyd George, composer Dmitri Shostakovich, children's author E Nesbit, aviator Amelia Earhart and scientist Albert Einstein.