The internets are full of condemnation for cheaters, and I often feel like I'm the only person tilting at the windmills of hatred against them.
Cheating is wrong. It's not the upstanding thing to do. It's an attempt to eat your cake, and have it too. If you're in a relationship where you're going to cheat, then you shouldn't have entered it in the first place. If you desire to experience intimacy that the rules of your relationship do not allow, there are two proper and upstanding alternatives: either to abstain, and reject temptation; or to confront your partner, and let the chips fall where they may. This could mean ending the relationship, or relaxing the rules.
(Or often, relaxing the rules, and then ending. That's how open relationships acquire a bad reputation; when closed relationships make their last-ditch efforts to save themselves by opening up, and can't.)
The thing is - when people cheat, we tend to judge them way too much. We vilify the cheater, and exaggerate the harm done to the person who was cheated on. We maximize their victimhood, and minimize what they did to help create the situation. Cheating is only ever seen as the cheater's fault.
I argue that the way most people set up their expectations in relationships makes it inevitable that someone will either cheat, or be tempted to. Further, even when temptation is repressed - when cheating doesn't actually happen - the fact that there's repression is a cost. It's a lost opportunity for someone to experience something beautiful.
If you're in a monogamous relationship, the first thing you should notice is that you're extorting your partner. Monogamy is a form of blackmail. Your partner loves you; but you don't love them back freely. The only way you will love them is if they do not love anyone else. Your partner has a burning need to be with you; but the only way you'll grant them this is if they agree to abandon other needs they have. You will not allow your partner to fulfill all their desires. You will not allow them to fully express themselves.
The most common response is that this is fine, because people agree to it. That marriage vows are entered voluntarily. But to suppose that this is fully voluntary is wishful thinking. There is pressure from peers and family; from churches, colleagues, and employers; from the state; pressuring people to at least appear to pursue a traditional, heterosexual, monogamous lifestyle. Even heterosexual monogamous people who decide to merely not have children can be treated as social pariahs. We are only now, and only in select countries, on the road to marriage equality for people with same-sex partners. For people with multiple partners, widespread acceptance is a long way down the road.
It takes unusual guts and self-awareness to go against this pressure, especially at a young age when a person still needs the support of others. Now add to this that a young person getting married doesn't necessarily yet understand themselves, or know what they can and cannot repress in the long run; add to it that the most valuable thing which they know they want - the love of their partner - comes dependent on a monogamous promise; and it's hard to argue the decision isn't made under duress.
There are certainly people who take shitty actions for no reason. In most cases, however, people make shitty decisions as a result of shitty circumstance. What hurts the most when you're being cheated on is the lying, the betrayal; but the main reason the person is lying is that what they have with you does mean a lot to them. There would be no reason to lie, if honesty didn't lead to losing something they value.
So then why cheat, in the first place? Well, because they don't only love you. They also love, and/or need, something else. In the situation they're in, there's no honest course of action that would allow them to have both. Either they lose you; or they lose the other something. It's a lose/lose proposition for them.
Cheating is what people do to delay an inevitable loss. For a while at least, as long as they can hide it, they can have both things they want - which they believe they could otherwise never have.