Open relationships are usually started for one purpose and one purpose only: nookie, said London psychotherapist Nancy VanderSchaaf, who deals primarily with married and long-term couples.

It seems like the perfect set-up. Partners get to have the stability of being in the relationship, but the variety of hooking up with different people like they're single again.

Nothing can go wrong, right?

Of course things go wrong! Twenty-nine-year-old Samantha (name changed) tried an open relationship with a former long-term boyfriend.

"The whole thing was very confusing," she explained. "It was a last resort to keep the relationship going after being in the relationship for a long period of time."

"For me, I don't think it works."

VanderSchaaf agrees that the majority of cases she's seen where making the relationship open became an option usually ended in failure.

"Jealousy gets in," she said. "And these are people who say they don't get jealous."

Oftentimes, even if the couple agrees to make the relationship open, one partner will inevitably develop insecurities about the situation. It could just stem from having to share their partner, or from a legitimate issue, like their partner has expressed real feelings for someone outside of the relationship, VanderSchaaf explained.

It's not even really about the physical relations with other people, it's about security, she added. For example, one couple she worked with had lived together for 20 years, then got married but maintained their open relationship. VanderSchaaf asked why they bothered to get married, and the answer was about security, which they felt marriage gave them. "It became more official because someone 'out there' recognized it."

"It's really all about security and safety," she said.

If a couple wishes to pursue this type of option to spice things up, it's imperative to define limitations. Both parties must agree, and it must be determined whether it's just for sex or for other reasons, although the reason is probably sex. VanderSchaaf cited one couple where the husband had a "traveling buddy" because the wife didn't want to travel — and sex was involved.

However, Samantha said she believes that if the relationship comes to the point where an open relationship becomes a possibility, it may be time to reevaluate it instead of trying something drastic.

"If you're in a relationship that gets to that point, look at the relationship before getting others involved," she said.

"It's like communism — on paper, it could work. When you're dealing with people and their emotional baggage ... it can lead to some pretty messy situations."