1) I don't know why everyone is assuming that the cheated on spouse is a "her". In case anyone cares, this isn't about me. I chose divorce (and I never cheated).
2) Part of the problem with marriage is that it lasts unreasonably wrong. People change in 10 years (let alone 50 years). Many people are in love when they marry and intend to stay that way for the rest of their lives... only to find that lifelong love is an unnatural thing (especially in this day and age where we live so long).
You can tell someone at 25 that you shouldn't marry if they don't plan to stay together forever, and they can believe you. But after 10 years and a couple of kids, an awful lot changes.
3) Parenting and Marriage are intimately connected. Kids do suffer when their parents get divorced.. this is well documented. Staying with a spouse even after the love is gone for the sake of the kids is not an unreasonable thing.
4) Lifelong monogamy is unnatural. Any human who denies this is lying. The fact that we have cultural institution that pressures us into lifelong monogamy is a relic of earlier days. It doesn't work.
Many cultures that have the institution of marriage didn't pressure people into lifelong monogamy. Marriage provided economic stability and a stable environment to raise children, but having sexual partners on the side was tolerated. Even in modern countries from Spain, to Denmark to Thailand adultery is treated much less seriously than it is in the US.
Our unreasonable insistence on absolute monogamy is likely the fault of our Puritan heritage. If the society restricts each individual to one lifetime sexual partner, it goes a long way to reducing sex other than the amount needed for procreation.
5) I understand the point about honesty. This is a good point.
6) The point I am making is about lesser harm. If a working marriage can be saved, and children can be spared the pain of divorce (not to mention the economic cost of living separately and the legal cost of untangling possessions and finances)...
Shouldn't doing whatever is necessary to prevent a divorce at least be considered?