I'm a bad example for answering this question, as I tell my SO he can sleep with all the women he likes, as long as I'm "First Wife". :happy: (yay attractive emoticons!)
However there was a time this was not the case, and with this previous SO, I did find out he was having an affair. Did I care? I was shocked to realize I did not.
Ultimately, I don't care what my partner does with other people. I care how they treat me directly.
I think most of the hurt regarding "cheating" comes from the misleading concept that love is finite and that sex and love have something to do with each other, when in fact they merely complement each other wonderfully. Most believe that if our partner is willing to be physically intimate with someone else they don't love us, when in reality seeking that physical intimacy outside the monogamous relationship is rarely a question of love.
A great deal of anger comes from feeling publicly humiliated or shamed by others who will judge us because our partners aren't faithful. There is a social stigma that says that somehow they failed as a wife or husband. Too often "cheating" is seen as a result of failure, when those perfectly happy in their relationships get wanderlust. There are many who still sincerely love their partners, but also enjoy, and to some extent genuinely need, interaction with others.
Ultimately, we are only trying to find pleasure and happiness to enrich in our short lives. For different people, this means taking different paths. But I do believe that in a world where people could be honest and count on understanding, most would choose honesty.
I sincerely believe there would be much less dishonesty in a world where divorce laws couldn't be used to abuse the partner. If partners (men or women) knew they could walk away from a relationship with a fair deal, and not have their spouse use the courts to abuse them and drag out the suffering, most would probably end things when they realized they couldn't keep their promise of monogamy. I really don't believe men or women should feel guilty when they stray, though they should be aware of possible consequences and be willing to face the music should everything come to light. Never risk what you aren't willing to lose.
There are two basic reasons romantic partners stray; either, the relationship is lacking in key aspects and they're in too deep to walk away; or, they realize polyamorous inclinations exist long after they were married. I believe that people in unfulfilling relationships like these would like to start with a clean slate, being honest with themselves and their partners.
However, we live in a world built to favour monogamy and normalize it, and one that makes ending a marriage excruciating to deter such action. If marriage is an institution and not about love, then I believe it should be treated as such. People naturally drift into monogamous relationships because they think that's the formula for life. Monogamy is viewed as the only proper course and people don't see any other real viable option for their life.
There is no doubt in my mind some individuals are fully capable of carrying out a faithful relationship, utterly uninterested in anyone but their partner, while others are simply instinctively territorial with their mate. Others let monogamy become about exercising power - interestingly I think women are most often guilty of this.
In my experience this is not the most widespread view. Most good people are complex, and there are many reasons why we live our lives the way we do. If people willing to have multiple encounters and engagements could find others they could be honest with, I believe many people would choose that path. In a world where having multiple partners was viewed to be as normal as monogamy, those prone to different lifestyle choices likely wouldn't even make a first, traditional date.
However, in this world, it's hard to find each other. The world has plenty of resources to help married couples succeed, but shockingly little is offered to those seeking an independent or poly-amorous lifestyle. They are told again and again that they are incapable of love, they haven't matured or there's something wrong with them.