Matthew constantly cheats on his girlfriend. Everybody knows it — everybody that is except her. We wish he wouldn’t because we are fond of her, but at the same time we all accept this as simply part of who he is. We grumble and gossip about him — and the guys are worse than the girls — but not one of us has ever outed him. We can’t — he’s our friend.

Should you interfere in other people’s relationships? My gut feeling is no. On the one hand, nobody really knows what goes on between two people and frankly it doesn’t matter, because whatever the case, no consenting adult is obliged to explain or ask your permission to conduct his or her sex life in whatever way they might wish; it’s none of your damn business. On the other hand, nobody likes to see an innocent party get duped, deceived and lied to either.

I conducted a minor survey amongst friends and acquaintances to see what they thought. Like me, most people reckon you’re better off staying out of it. Karen, however, was the first exception. She thought confronting the cheat was the best option. “I’d give them a couple of weeks to tell their partner themselves. If they didn’t then I would do it and deal with the following shit storm,” she said.

Nik, however, thought that was a terrible idea. “I’ve been cheated on and that sucked. I’ve cheated and that sucked too, but it’s nobody’s place to make judgement calls unless they are one of the people in the relationship.

It’s no-one’s right to pressure one member of a relationship with an ultimatum before they expose the cheater’s perceived evil deeds themselves. In fact, that could be a quick way to lose the very friend you think you’re saving. Are they going to believe you? And what are they going top make of the fact that you have butted into their relationship?”

Nik highlighted an interesting and unfortunate side-effect of interfering — the messenger may be the one who gets shot. The fallout from an imploding relationship can scatter far and wide, and interfering is an excellent way to get caught in the blast zone.

Way back in the mists of time when I was but a slip of a girl, a young man of my acquaintance jumped off a twenty storey building when he found out his girlfriend had been cheating on him. Granted, this was an extreme response and one most of us would be unlikely to make – I was not in any way involved in these tragic events but since then I have had an almost pathological desire to mind my own business.

The easiest answer is to do nothing. But, as my friend Amanda noted, this too can damage a friendship.

“I was friends with a couple who were married and had a kid together as well. I used to see the husband more often as he worked in entertainment and work and social life tended to overlap. Then he started introducing everyone to his girlfriend and even took her on tour. I didn’t know whether his wife knew about her or if they had an open relationship. I was in my early twenties and, for some reason, I was just too uncomfortable to ask! I didn’t want to look like a prude if they were in an open relationship. I didn’t want to bust my cheating friend but I also couldn’t condone lying and cheating. I felt torn and it really strained my friendship with both of them to the point where I lost contact.”

So could it be that you are damned if you do, damned if you don’t? Perhaps…

Perhaps the more interesting issue is not whether or not to expose a cheater, but why we hold the idea of sexual fidelity in such high regard. Evolutionary psychologists have theorised that it is because no man wants to waste his labour and resources raising another man’s child; and that women fear a cheating man will deprive her children of the very same labour and resources, thus imperilling their chance of survival.

It is an interesting idea but one that doesn’t really stand up – otherwise nobody would adopt or marry a man or woman with children from a previous relationship. Besides which, most people regard cheating as anathema, whether or not a relationship has produced children — or even whether or not a couple are married. Which is bizarre, considering you haven’t stood before God — or a judge — and your friends and family promising to forsake all others.

Sometimes the fear of possible infidelity verges on the hysterical. When DNA testing first became available to the general public, there was a torrent of articles claiming as many as 30% of children had not been fathered by the person whose name appeared on their birth certs. The most quoted figure was one in ten children, but subsequent studies — not media scares — found that the figure varied between one and three percent, depending on where you were in the world. Women may be faithless Jezebels, but we are unlikely to present you with a cuckoo despite that, just as most married men do not leave their wives after an indiscretion or three.

Perhaps most interesting of all is that although monogamy is widely touted as the ideal, many people fall short in relation to it; and almost as many are in their own discreet way, not particularly concerned about it. Scratch the surface behind the ideal and you’ll find that far more couples than you’d expect are not living monogamous lives, whether or not they have polyamorous or open relationships, or simply decide to leave the door to their relationships ajar.

If there is one very good reason not to interfere in anyone else’s business it is that relationships are about more than sex. They are about emotional support, family, companionship and love — and having sex with other people does not negate any of those things. As wonderful as it is, sex is not everything.