"Open marriage destroyed Ashton and Demi's relationship!" cried one tabloid. "Did Ashton and Demi have an OPEN MARRIAGE?" spat another. When Hollywood couple Ashton Kutcher and Demi Moore split last month amid rumours of having an alternative union, the press had a field day. The astonishment and bewilderment over a couple engaging in such a lifestyle was screamed from the front pages.

We live in a society that is more sexually liberated than ever before, yet open relationships – a relationship in which both partners are allowed to have sex with other people – still have the propensity to shock. It is one of the last remaining taboos.

Kutcher and Moore are not the only high-profile couple to allegedly reject monogamy. The actress Tilda Swinton caused similar ripples when she gave a frank interview in 2008 explaining her unusual living arrangements. She and her long-term partner, the artist and playwright John Byrne, have been together for more than a decade and live with their twins in a rambling house in Scotland... along with her 33-year-old lover Sandro Kopp. When pressed, she remarked: "We are all a family. What you must also know is that we are all very happy."

Mo'Nique, the actress and comedienne who won an Oscar for her role in Precious, and her husband Sidney Hicks are another couple who have spoken candidly about their personal life. "Could I have sex outside of my marriage with Sidney? Yes. Could Sidney have sex outside of his marriage with me? Yes. That's not a deal breaker," Mo'nique told a flabbergasted Barbara Walters in an interview.

So can open relationships work? Jenny Block, the writer and author of Open: Love, Sex and Life in an Open Marriage, certainly thinks so. The 41-year-old mother of one is currently the poster girl for open marriage in the US, describing herself as "the most average-looking, regular soccer-mom type". Having married her husband, Christopher, in 1997, Block embarked on an affair with another woman three years later. When she finally came clean to her husband, she found his response fascinating. "What was so interesting to me was that he said, 'I can't believe you lied to me', rather than, 'I can't believe you had sex with someone else'," she says.

"It was the trust thing rather than the sex thing that had hurt him and so I began to ask myself which was more important and what was marriage really based on?"

They decided to embark on an open marriage, albeit with certain ground rules: complete honesty and strictly no carrying on with someone else from their neighbourhood. Currently Block has a girlfriend, Jemma, who has her own apartment but is also considered part of the family. While Jemma and Christopher don't have a sexual relationship, he is free to date other women. Keeping up?

"We're not monogamous creatures, it is a lifestyle choice," Block says. "And it doesn't always work. In the US we have a 50 per cent failure rate for marriage. If you get 50 per cent on a test at school you wouldn't be like, 'Great! Keep doing what you're doing'; you'd look for better results."

What irks Block is that we live in a society where cheating is acceptable (if not exactly welcomed), whereas open relationships are scrutinised. "Isn't it better to be honest about your desires?" she asks. "I'm not claiming that this is possible across the board or that we're all ready for this yet, but I'm suggesting that this is something that works for us and other people."

Her 13-year-old daughter is aware of the situation and the couple have elected to answer any questions as they come. But Block stresses that theirs is not some wild household with people swinging from the chandeliers. "We couldn't be any more mainstream if we tried," she says. "Saturday night is Scrabble and Chinese take-out."

As one might imagine, Block and her family have been given a hard time by certain groups since the release of her book three years ago. Why does she think people object to her situation?

"I guess it's hard any time you stray from the social norm. Our society's foundation is based on monogamous, heterosexual marriage," Block says. "But even though we have had people be unkind sometimes, unless we all sort of come clean, when are we ever going to get a conversation going?"

Despite Block extolling all that open marriage has to offer, it shouldn't come without certain warnings; jealousy being the most obvious catalyst for causing cracks. "It really depends on the couple and what their values are but generally it doesn't work because eventually somebody will form an outside attachment and that will cause problems with the primary relationship," Mandy Kloppers, a relationship psychologist and counsellor, says.

On her decades-long relationship with Jean-Paul Sartre, the philosopher and feminist Simone de Beauvoir wrote: "We were two of a kind and our relationship would endure as long as we did: but it could not make up entirely for the fleeting riches to be had from encounters with different people." It was Sartre who proposed the open relationship and he was the one who engaged in numerous affairs, while de Beauvoir rarely did. Critics have observed that her fiction, so autobiographical in nature, suggests she suffered deeply from jealousy, going along with Sartre's plan merely to please him.

This, Kloppers points out, is often the outcome of such arrangements. "It's common to see one person coerced into it because they want to keep their partner happy and want to keep an eye on them," she says. "If you have an unstable relationship to begin with then you're asking for trouble by doing this type of thing."