It's late, the booze is flowing, the woman is beautiful. Flirting and kissing are followed by a hook-up. All of that wouldn't be a problem if Max wasn't married. His wife doesn't know about her husband occasionally having sex with other women. The sexual partners don't mean anything to Max, he says — he's merely letting his sex drive take over every once in a while.

"I know it's a natural thing," Max said to DW. "But it's definitely wrong as well."

Gertrud Wolf is a couples' and sex therapist. "Humans aren't monogamous by nature," she says.

Roughly one third of Wolf's clients come to see her because one of the partners cheated. For most couples, sexual fidelity is very important.

"I often work with people who had always said that they'd never cheat, and that they could never forgive their partners if they would," Wolf told DW. "And then they sit here with hanging heads because they did cheat after all."

Many people — especially young ones — wonder whether monogamy really is the only meaningful form of a relationship. Some consider it outdated and predict its imminent demise.

"Monogamy is a cultural achievement of man, an invention," Wolf said. "But one that makes sense."

5 declarations of love that'll move you to tears Gunter Sachs' 1,000 roses for Brigitte Bardot The French femme fatale and the most famous playboy of the day seemed to have been made for each other. In 1966, Sachs, the German son of an industrialist, showered the famous blonde actress and singer with 1,000 red roses in a dramatic marriage proposal. Though the couple divorced after just three years, the romantic gesture remains legendary.

5 declarations of love that'll move you to tears The Royal Albert Hall from Queen Victoria It was an arranged marriage, but Queen Victoria quickly fell head over heels for her husband, Albert. The couple was inseparable up until Albert's death in 1861. A decade later, Victoria commemorated her partner by opening a concert hall in his name, the world-renowned Royal Albert Hall in London. This family portrait was painted in 1846.

5 declarations of love that'll move you to tears A double-decker bus Ken Morgan was a former bus driver and Shirley Morgan had been a conductor. They met over 61 years ago in a double-decker bus in England and it was love at first sight. They are still married after all these years - and still seem to be very much in love. As a sign of his affection, Ken presented Shirley with an unusual gift: He bought the bright red bus they had first met on.

5 declarations of love that'll move you to tears Sinead O'Connor's tears of love The tears streaming down the cheeks of the Irish singer will give you goose bumps. In the 1989 video "Nothing Compares to You," she sings about a lost love - one of the most emotional declarations of love in music history. The tears at the end of the video are real, O'Connor has often maintained - although they weren't shed for a man but for her deceased mother.

5 declarations of love that'll move you to tears Cinematic moment in 'Life Is Beautiful' In the comedy-drama by director Roberto Benigni, Italian Jew Guido and his wife and son are deported to a Nazi concentration camp and separated. His declaration of love for his wife Dora at the end of the film moved millions of viewers to tears. Over the loudspeakers at the camp, he said goodbye to his "principessa," sending her a message of hope. Pictured above is the German film poster. Author: Antje Binder (ct)



Two forms of sexuality

In her scientific book "Construction of the Adult," Wolf describes two forms of sexuality that often get in each other's way.

The archetype of sexuality is a mere physical need. It is not about closeness or intimacy, but about satisfying the urge.

Max knows that. So does Rebecca. The 30-year-old says that she experimented a lot in her early 20s. She went to swinger clubs, had many adventures and really enjoyed the excitement.

The second kind of sexuality is sex that creates closeness, or "sexuality as a form of culture," as Wolf calls it. For her, monogamy plays a central role here because it strengthens the bond between two adults.

Wolf explains why sexual fidelity makes sense within a partnership — at least if you want a deep, trusting relationship: "You can't divide your time between a limitless number of dating partners if you want a serious connection," she says.

Oxytocin turns sex into more

Max is always drunk when he goes to bed with other women. He doesn't sleep with any of them more than once, because intimacy and closeness are actually the last thing he's after.

Maybe his body somehow blocks the path of oxytocin, the "connection hormone" released during orgasm. It's the same chemical that also floods into mother's bodies during breastfeeding, strengthening the mother-child bond. Sex, even when it starts out as something with no meaning attached to it, therefore always carries the danger that at least one person involved ends up wanting something more.

Read more: Does the penis ever get bored?

People who live in a monogamous partnership try to overcome this danger by simply prohibiting sex outside the relationship. Rebecca, too, after her eventful past, has now arrived back at a monogamous relationship. She describes her boyfriend as rather conservative, as someone who doesn't share her joy of experimentation.

"I don't miss not being able to have other men," Rebecca told DW. "But sometimes I miss the excitement."

Rebecca hopes that at some point her boyfriend will also long for more freedom and will at least take their sex life out of the bedroom, which feels like a cozy cage to Rebecca. But what if this desire isn't fulfilled?

Another third of Gertrud Wolf's clients come to her because their sexual needs seem incompatible.

When one partner wants more, or different, sex than the other one does, then the appeal of sex with strangers finds more fertile ground — and cheating no longer seems like that much of a stretch.

A relationships can emerge stronger from a crisis, if both partners are willing to work on it

Wolf says there's no one solution that fits all couples across the board. For some, it can make sense to open the relationship for a while and let go of sexual exclusivity. That's exactly what Kathrin and her boyfriend did.

The couple decided that sex with others should no longer be forbidden. Kathrin is aware that this experiment comes with risks and that it isn't just fun and games.

"To me, this open relationship doesn't mean I can do whatever I want," she told DW.

Because there is no longer a ban that defines borders, Kathrin says, she must take responsibility and decide what she can and cannot do while staying true to herself and her partner.

An affair is also an opening of the relationship — except it's one the cheater's unwitting partner has never agreed to.

"Whether it's an affair or an open relationship, all of this can lead to crises in a partnership," Wolf said. "But these crises can also lead the partnership to grow."

Monogamy not 'the only meaningful relationship concept'

The therapist insisted that "monogamy is neither the only meaningful relationship concept, nor has monogamy failed."

Which form of relationship makes sense for two people can change again and again and must be decided by each couple individually.

"Of course, sexuality is very personal," Wolf said.

But in a partnership, she added, sexual decisions are no longer made by just one person. They become a matter of negotiation.