I don't think that there is any absolute duty on the part of others to inform someone they are being cheated on. I'm not claiming the reverse -- that one has an absolute duty to refuse to meddle. This is a difficult judgment to make at the best of times, and it could go either way.
As a rule of thumb, I would recommend not meddling, since an indiscretion may be impulsive and temporary, and the guilty party may regret the indiscretion and resolve to make the original relationship work. Once the partner learns about the indiscretion, the relationship is likely doomed with no chance of repair.
eudaimonia,
Mark
This. It's a judgement call. Yes, most everyone would say that they would want to know if they were being cheated on. But since when do people want what's good for them?
I look at this from the perspective of a member of a small town where my mother knows everyone and everyone knows her and everyone tells her their dirty little secrets, because she is so good at keeping them and never tells anyone except me, because I now live two hundred miles away and I don't have anybody to tell.
And let me tell you, more bed hopping happens in small, quiet towns than you would ever believe. A lot of people have small affairs, and it often doesn't make a difference how happily married they are or how many kids they have.
Studies have shown that the majority of men who cheat on their wives are not unhappy with them. And while it's slightly different for women, who tend to cheat when they are
dissatisfied with some aspect of their monogamous relationship, it doesn't always mean the end of said relationship. Sometimes a midlife crisis or a fling is just that--a temporary situation, after which they go back to married monogamy quite willingly.
So here's what I've observed myself and heard about from my mother. A great number of the people we know have had affairs. Of these people, the ones who told their spouses have gotten divorced and had to deal with moving out, splitting time with the kids, disentangling their finances, and the lengthy and often vicious lawyer-fueled divorce proceedings. The ones who haven't told their spouses are still married and happy with the person they married.
Some people might say the happiness of the cheated-on spouse is a lie. Maybe that's true. I don't know how happiness can be a lie, or whether it being a lie makes a significant difference to the person who has it. But I do know that the pain of finding out your spouse is cheating/has cheated and going through a difficult divorce IS real. And the unhappiness of the children involved is real too. And if both partners were happy in their relationship, but one of them made a mistake/wanted a bit of variety/got seduced etc., is that really worth ruining an otherwise good marriage?
So it's a judgement call. For Mom and I, we keep out of it*, impartial observers, because we've seen this sort of thing go both ways and you can never tell. People say they would want to know if they were being cheated on, but really why? We don't want to think about industrial chicken farms when we're enjoying a bucket of KFC. We don't want to think about abuse and slavery when we get a diamond necklace as a gift. We don't want to think about how just about every time we eat something red, it's been dyed that way with 'natural' dyes that were probably made from crushed bug exoskeletons. When we fill up our tanks, we don't want to think about how every gallon burned means one less gallon will ever exist until there are no more gallons at all. We ignore things that would make us unhappy all the time. Maybe we shouldn't. But we do, and most of us are quite happy to do so. Is marriage really any different?
If people already seem unhappy in their relationship, and one of them is cheating, it would probably be okay to tell them. But if they seem pretty happy, don't mess that up. People aren't perfect, and a marriage that lasts a lifetime is going to have to survive a lot of stupidity, selfishness, bad judgments, dry spells, and mistakes no matter how much they love each other.
*Except in one case, where a certain husband is a total rake and either has tried or is trying to bone just about every woman in town. In that case we have quietly spread the rumor that he has an STD to hamper his efforts. For all we know, it's true. 