91% of Americans think adultery is morally wrong.
Americans have become more accepting of divorce, pre-marital sex, having babies outside of marriage and gay marriage, so why do so many hate the idea of adultery and cheating? The number of those who disapprove is almost twice as large as it was in the 1970s.
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>>