I think it all boils back down to the needs not being met (usually psychological). Every reason people can give, to me in my mind after having psychology shoved into my brain from a young age, comes back down to that.
In my mind, also, it is always the cheatERs fault. They may blame it on their SO- he isn't good enough in bed, he cheated on me first, he told me the dress didn't flatter me- but in the end everybody has to take responsibility for the sex they have. In conditions of rape (I can't call that cheating- it's not if you don't give some consent) obviously that's different, but when people go out and get roaringly drunk and cheat, I still think that isn't okay. When you take alcohol, you should be aware of what can happen. I think that's a responsibility. If you aren't willing to take ownership for what you do when you choose to drink or take other mind-altering substances, then I don't think you are mature enough to do them in the first place.
But I've found most people do things like that when something psychologically isn't right. They may not communicate their bedroom needs or their emotional needs. They may not even know their needs themselves. But generally, excuses like "revenge" or "I was drunk" or "I need more sex" just cover up the real problem. And there is a real problem. I think part of the problem is that many people these days (at least in my generation) seem to believe in some mythic "perfect" mate that will automatically make everything ideal. They don't realize that it doesn't exist. Nobody can just know what you need at any given time. You need to communicate with your partner! Nobody can mind-read, and this is what really irritates me about "soul mates." Some kids think if somebody is their "soul mate" then they'll just "know" what they need, and if they are committed in a relationship and a small problem happens
Sexual addiction is a legitimate psychological problem (and physiological too). But, everybody I know with that kind of a problem can't even form a lasting relationship for them to cheat. They know they can't be monogamous so they don't try. Granted I don't know many people who fit into that category.
I have a bit of a "lenient" policy myself on cheating. Personally I would NEVER do it- if I'm unhappy for whatever reason or feel like I "can't" control myself, I would break things off- and as I've said somewhere else on here before, I love my current boyfriend and have full intentions to marry him. I've also only been cheating on once. But for me, if I ever do get cheated on, I don't dump them. I don't see it as something people do for shits and giggles because "they can." I'd sit down and talk about what it is that's missing from the relationship. What is it he needs? Is that something I can provide? Is it something he will accept from me?
My dad has not been faithful to my mother most of my life. It's not that he goes away on business and hires hookers or something like that. He just cultivates these other relationships because he doesn't understand some vital pieces of himself. He doesn't know what a good "wife" or "mother" is. He grew up in a home where his own mother did not care for them at all. He was the eldest, he had no structure, he had nobody to help him process emotions. He loves us, and I don't think his infidelity was caused because he "thought he could get away with it." It wasn't about fun; it wasn't about addiction. It was about my mom working a lot and my dad being in unfamiliar places and having no idea what to do with himself emotionally or how to communicate it, and instead believing these people that could give him momentary stability would "fix" the problems. Instant gratification. Sorrow after. Pain. Not understanding what is going on, at all, in your world. Being unhappy in general and thinking that other people are responsible for your happiness instead of realizing your own power. Thinking your wife/partner is responsible for your daughter's emotional problems and the pain you see in her. It's a mess, honestly. It's usually very complex.
Personally I think treating those who cheat horribly is the worst thing you can do for them. They aren't all master manipulators just doing whatever the fuck they want to. I honestly believe most people believe themselves to be good or believe they are doing their best to be good. I think "repeat offenders" are repeating because they don't get the benefit of talking it out. We seem to have this "no tolerance" policy where if you catch them, they're instantly out- that doesn't go down deep enough into their psychological problems to help them understand HOW to do relationships.
Dang this is long. I guess what I'm trying to get at here- "Hurt people hurt people." There's something inside of those who cheat that needs repairing. They cheat because they don't know how to make monogamous relationships work. Simply ending a relationship, no questions asked, will only further their "cheating" because you aren't helping them understand how to work a long-term monogamous relationship. It isn't a "punishment" that will inflict change because they don't have the "alternative" to understand. They don't have other ways to cope with their desires and urges and confusion. You have to sit with them and process it out, help them understand their needs, and then what is a need and what is a want, and what a partner can and cannot just do, and how long-term relationships actually *work*.