What sort of man would let his wife sleep with Brad Pitt? My husband said he wouldn't mind...

When my husband of four years tells people we have an open marriage, the reaction tends to follow a pattern. First they’re surprised, even shocked. Next comes curiosity and a flicker of envy. This is swiftly and inevitably followed by the realisation that the openness goes both ways.

“Wait! Does that mean she also…?” They trail off, their faces falling.

We all know people cheat, but the idea that a man would allow his wife to sleep with other people is taboo to the point of unthinkable for many. Which is perhaps why the news that Brad Pitt’s alleged new girlfriend is married to someone else raised so many eyebrows.

27-year-old German model Nicole Poturalski, who was pictured getting off a private jet with Pitt in the south of France last week, is said to be in an open marriage with her 68-year-old husband, German restaurateur Roland Mary, with whom she has a seven-year-old child.

Mary is described as a “free spirit” who is “not interested in negativity or jealousy”. Relationship equality has certainly come a long way, but it’s still hard to imagine a man whose insecurity would not be piqued by Pitt.

Similarly, shock over the resignation of government scientist Neil Ferguson in May after he broke lockdown rules to meet his married lover – who lives with her husband, although Ferguson claimed they considered themselves “one household” – was laced with incredulity that any man would put up with, let alone seek out, such a situation.

But the truth is, plenty of men are quite comfortable – happy, in fact – to share.

Roy Graff, who lives in London, was a one-woman man for most of his life before discovering ‘consensual non-monogamy’ after his divorce at the age of 40. He began online dating and met a woman who told him she was polyamorous. Graff describes it as a eureka! moment: “I’ve never looked back,” he says. “The idea that you can have multiple partners and actually talk about it and be honest was amazing to me.”

Now 50, Graff has two partners, who he says are completely equal in importance, and runs a mentoring service called Open Relating for others interested in doing the same. Both his girlfriends have other partners and he admits to jealousy on occasion, but says nothing either does with someone else detracts from how much they love and value him.

“I do have pangs of jealousy if I see my girlfriend kissing another man but I just remind myself of all the good things we have together,” he says.

UK statistics on open relationships are, not surprisingly, hard to come by, but a report published in the German Journal für Psychologie suggests around 4-5 per cent of American adults are currently in consensually non-monogamous relationships. A YouGov survey earlier this year found that 32 per cent of US adults say their ideal relationship is non-monogamous.

When Billy Procida, 31, from New York met his girlfriend Megan ten months ago, she’d already been in another relationship for two years. While his family and friends are respectful, if a little baffled by his choices, Procida’s public profile as a comedian and podcast host means he often receives online abuse and derogatory comments about Megan.

“I’ve had so many people tweeting me saying they can’t believe I’d let my girlfriend be with another guy,” he says. “But she’s not ‘my’ girl. I don’t own her.”

Like Pitt, he has had to negotiate what it means to date someone who is already in a longstanding relationship with someone else.

”It didn’t faze me, but it felt important to be friendly with him,” he says. “Every time I was at their house and he was home I was trying to be on my best behaviour.”

Many, like Graff and Procida, happen on non-monogamy almost by accident, but Steve Cook (who asked The Telegraph to use a pseudonym to protect the identity of his family) and his wife of four years, who live in the south of England, always planned to close their open relationship when things got serious.

As time passed, however, they discovered they were both pretty happy with the status quo – six years on, they are married with a young child and both remain free to see and sleep with other people.

Why marry at all in that case, some might wonder? Cook, 39, says they still wanted to make a lifelong commitment of love and partnership to one another, which he doesn’t believe is diminished by seeing other people – though their families are unaware of their arrangement.

“I think all couples define marriage in their own way. We saw our relationship as the central, connecting thread that ran through both our lives.”

He claims his jealousy would only be sparked if his wife started prioritising another relationship over their marriage: “This has to take precedence, especially since we had kids.”

He has a partner he sees around once a month as well as two more whom he sees a few times a year. His wife is less interested in romantic relationships but enjoys having sex with other people at parties, something he says he doesn’t mind.

“I don’t have any hangups about that at all,” he says. “My wife and I have a life together and we’ve built a strong relationship and her having sex with someone, even Brad Pitt, wouldn’t change or challenge that.”

Nevertheless, given the potential for pain, you might be forgiven for wondering whether both parties in an open relationship are always equally on board.

“Some people may agree to something they don’t really want in order to keep the relationship alive because it is more tolerable for them than breaking up,” agrees psychosexual and relationship therapist Silva Neves. “It isn’t a good recipe for a happy relationship but it is quite common.”

He notes this behaviour is equally prevalent among men and women.

People often think of men as being naturally promiscuous, which explains why women such as Poturalski come under more scrutiny. In her 2018 book Untrue, anthropologist Wednesday Martin drew on research in both primate and human female sexuality to conclude that, actually, women need just as much sexual variety as men.

Open relationships, by their nature, won’t work for everyone. But those that choose them say they offer freedom, excitement, and in some cases, greater security. “In polyamory, I know that if a woman wants to be with me, it’s because she wants to be with me, not just the idea of a man who provides for her,” says Graff.

When I ask my own husband how he’d feel if Brad Pitt asked me out, he is pragmatic.

“I wouldn’t like having the paparazzi outside our house,” he admits – then pauses, thoughtfully. “Actually, if my wife can get with Brad Pitt but also wants to be with me, that speaks quite highly of me. It elevates me to a league I didn’t know I was in!”