91% of Americans think adultery is morally wrong.
One theory is that marriage is now seen as a "capstone" rather than a "cornerstone." A "cornerstone" marriage is embarked upon by younger people who must struggle through problems yet still make it through. Adultery might be considered one of those problems.
A "capstone" marriage is a marriage where people are older and wiser and have gotten all those problems out of the way. It's a marriage where both parties should be mature enough to be honest with each other, which means no hanky-panky.
Interestingly, the abhorrence towards cheating has risen while the acceptance of alternatives has also risen:
The same Gallup poll that found near-unanimous disapproval of cheating also found rising acceptance of many other non-traditional, consensual sexual relationships. The new ethical consensus that you can do whatever you like as long as you're not hurting anyone—and as long as you're being rigorously candid—reflects a thoroughly modern mix of tolerance and puritanical censoriousness. We've become more willing to embrace diverse models of sexual self-expression even as we've become ever more intolerant of hypocrisy and the human frailty that makes hypocrisy almost inevitable.How Marital Infidelity Became America's Last Sexual Taboo, The Atlantic>>