1) There is a logical flaw in your argument, Engineer. The agreement I made (and that I think most people made) is to love my now-ex-spouse "until death do us part".
Divorce violates that agreement in the most drastic way possible.
2) You are misstating my position when you say "one should move on and leave the other to languish." Marriage is not only about sex. There is a stable family unit. There are economic benefits to sharing a house hand splitting time with children. There are social benefits to being together.
Having an affair, especially a short affair over a limited time, is not "moving on". It is a temporary thing that allows most of the marriage to continue with the benefits unaffected.
2) My position is evolving a bit... or at least it is being clarified for me. I think that my point is that the American institution of marriage in the 21st century is broken. (I pointed out that the way marriage and monogamy function in American culture is different than in other cultures.)
Let's go with your business metaphor.
Most of the time one party violating some term of a business agreement does not lead to a divorce. There are many times there are violations of the letter of a contract that no one pays attention to. When one side finds a violation that they do care about, generally there is some agreement or restitution or often just a pledge no stop doing whatever... companies understand that most of the time, completely divorcing from a business agreement isn't worth it.
If there are problems, you work them out. Divorce has cost.
3) I am interested in the sociological meaning of marriage, and how it is broken....
Marriage as understood by 21st century Americans is a strange mix of religious precepts and solutions to economic problems found in the 19th century.
In the 1800s women didn't work very much outside of the house, the life expectancy was about 50, and the economic needs of supporting families meant that breaking up families wasn't an option.
In those circumstances... a lifetime pledge of monogamy perhaps made sense, and anything that threatened the stability of the family unit (i.e. unfaithfulness) was treated as a grave crime. Notice that other cultures developed different attitudes.
Does the way we practice marriage, as a lifetime pledge of monogamy that many (if not most) will break either by divorce or by infidelity, make sense any more? Sure some people are lucky enough or virtuous enough to do it... but it clearly isn't natural for the majority of us.