The first 2, although not great, do happen. The 3rd is just unacceptable. People who do this (I'll call them sharks) feel somehow that the rules do not apply to them. They have learned over time that if they bend or break the rules that they get away with it just fine. They begin to think of themselves as "gamblers" (but who always win). They don't "play by the rules". They "just do it".
Why do these sharks play with the other fish? The ones who live life by the Golden Rule (remember that one? - loosely, treat others the way that you would like to be treated). Because they can't handle other sharks!! The sharks like the security of the "neurotic housewife" (be it male or female) who provides their security blanket while they play their little cheating games. But will they share this information about their lifestyle with you, so that you have a chance to say "thanks but no thanks" and move on, or perhaps set up your own little episode of romantic fulfillment? Oh no. Why? Because they fear all of the things that you are experiencing. That makes them hypocrites. Here's what happened:
Some key developmental milestones:
The fact that divorce is an accepted part of our culture has changed everything. Consider an analogy with dental care. Years ago, dental care was poor, and if you kept your teeth, people said you had good teeth, and if you lost your teeth, oh well you had bad teeth. It was the same with marriage - if you had a good one, lucky you, but if not, oh well. In either case, as my great-grandmother used to say: "you made your bed, now lie in it".
Due to improvements in dental care, there is no longer any reason to not have healthy teeth into old age. It is incumbent on all of us to keep our teeth clean and in good shape. It's just one more responsibility added to the burden of being a modern human being.
Likewise relationships. In the modern era, divorce is most certainly an option. Leaving your mate is definitely possible. Counseling and therapy are available. So there is no excuse for staying in a bad relationship. Kids are not a good excuse. Lots of kids have divorced parents, and although it is not ideal, somehow they get by OK. Kids who are stuck with parents who despise each other don't have it so great either.
The modern American paradigm for marriage is this: you fall in love, but if things don't work out, or things change, you go fall in love with someone else. Now we can piss and moan about how people just throw up their hands at the drop of a hat and bail out on their responsibilities, but that doesn't change the fact that this seems to be the new definition of marriage. How much work you put into a relationship before you quit is an individual thing. However, if your partner is long-term cheating and refuses couples therapy, it is time to go.
There is just no excuse for long-term cheating in a relationship. There is no excuse for tolerating long-term cheating in a relationship.