But aren't monogamous contracts "unrealistic"? This claim makes no sense. If 50% of people who vow life-long monogamy keep their promise, what's "unrealistic" about it? Monogamy is no more unrealistic than hundreds of promises that we expect people to keep—to show up for work on time, buy lunch next time, pay their workers, or give dissatisfied customers their money back. In each instance, if you think the terms are onerous, refuse them. Don't say yes, then blame the fates.
But what about human weakness? Here I take a hard line: Human weakness is a choice, and it should be criticized, not excused. I'm particularly baffled when economists say otherwise. In what economic model is "lots of people feel tempted to do it" a reason to turn a blind eye?
Is the monogamy contract of marriage unreasonable? Bryan Caplan says that even if the expectation of life-long monogamy is far-fetched, we still ought to expect people to honor their their contracts:
He concludes: "I'm not telling anyone what kind of contracts and promises to make, but merely to honor the contracts and promises they've made. That's not too much to ask of human nature."
I agree completely that we ought to expect people to honor their contracts, monogamy-related and otherwise. But I also think that this expectation ought to come with certain provisos.
For one thing, we ought to work to avoid setting up social expectations and practices that encourage people to enter into badly thought out contracts. Additionally, we ought not let the expectation to honor contracts drive us to over-the-top punishment schemes. So while we ought to refuse to accept failure, we also ought to expect it to occur in limited amounts and seek ways to build in appropriate mechanisms to deal with it.
Take bankruptcy, for example: There's no way we should condone it; it shouldn't be socially acceptable to put oneself in an untenable financial position. But harsher bankruptcy systems aren't nearly as good at dealing with failure.
No doubt, it's a tough balance to strike. But it does suggest that laws that deal with dissolution of contractual agreements—divorce laws, for example (divorce is perhaps a sort of marital bankruptcy)—should be designed with speed and resiliency in mind rather than enforcing norms through punishment.