Catherine Hakim, a British sociologist, offered up to a conference of mostly American scholars the theory that U.S. divorce rates were so high because we expect marital fidelity. If we just accepted, as continental Europeans do, that some discrete affairs are inevitable in a marriage, we could have lower divorce rates, like the Italians.
As a sociological observation I have to admit that she is probably right.
Every possible social arrangement has its costs and benefits, which require trade-offs. Still, I think the greater benefits, both socially and personally, come from trying to reach the higher standard.
I do think that it is unreasonable to expect your spouse to feel like your soulmate at every moment of a long marriage. But sexual fidelity does seem possible for most people who make a public commitment to try it. And there are things we can do to improve their odds.