In movie Hall Pass, two married guys get carte blanche to do whatever they want to for a whole week.

Holly Hill believed so strongly that humans are not built to be faithful that she appeared on television and wrote books advocating open relationships.

The Australian author claimed that the only healthy way to have a modern relationship was to allow your partner to have sex with other people, and do the same yourself.

But after insisting that allowing her boyfriend to cheat three nights a week kept them strong, she now admits it destroyed them.

So what changed?

Hill became an “anti-monogamist” after an ill-fated fling with a married man in 2006. The man never intended to leave his wife, she revealed, but wanted to sleep with somebody else.

After a painful break-up, Hill was determined to use her psychology degree from the University of Southern Queensland to find out why.

She came to the conclusion that men were hardwired to need sex from more than one person, and that humans were not made to have exclusive relationships.

Hill shared that view with anyone who would listen, and shocked and angered many commentators as she expounded her theories on CNN and Larry King Live.

She wrote a novel, Sugarbabe, about her year-long adventures with older men, and another called Toyboy. But now her dogmatic opinion has completely changed, she told Grazia magazine in an interview.

Hill and her then-boyfriend, Phil (not his real name), drew up a contract to determine how they would operate.

They vetoed anything they felt would make them jealous – in Hill’s case, her partner taking other women on romantic weekends away or buying them gifts.

She told CNN in 2010: “One of the main things that I have learned is that a woman who negotiates infidelity with her partner is far more powerful than a woman who is sitting home wondering why he’s late from the office Christmas party.

“It’s better to walk the dog on a leash than let it escape through an unseen hole in the back fence.”

But rather than eliminating jealousy, Hill became completely paranoid, shedding 12kg in weight and obsessively comparing herself to the other women Phil was seeing.

She said: “I was staggered by the effect our infidelity was having on me. I’d committed myself to the belief that monogamy was outdated and to have to even consider I was wrong was incredibly tough.”

Her boyfriend, who had previously enjoyed watching her flirt with other men at parties, now said he felt emasculated by the situation – and last year they split.

“I was devastated,” said Hill. “When it was just the two of us our relationship was incredible. But we’d ruined it by being ‘unfaithful’ – ironically, the one thing I thought would save our relationship.”

The two remain friends and Phil is now seeing a woman who insists on monogamy – something Phil says has actually restored his confidence.

Hill, too, is dating somebody new, and is feeling optimistic about doing things very differently.

“Finally,” she says, “I feel like I can see a happy relationship with one man.” – Daily Mail