W. Brad Wilcox, a US sociologist who researches in area of marriage, had a piece [1] in The Washington Post last 
week, attacking the idea, which appears to be gaining ground, that marriage be sexually exclusive is passé.
Wilcox points out that there are a range of problems with marital infidelity; out-of-wedlock births, increased risk of sexually transmitted infections, lack of emotional satisfaction, and the emotional risk to children where there is, as Wilcox puts it “a revolving carousel of romantic partners in the home”.
Wilcox also makes the point that popular approval for marital infidelity has actually fallen since the 1970s, despite the fact that marital breakdown and divorce has increased, and the number of people getting married has fallen.
According to Wilcox, fully 79 percent of American adults now saying that infidelity is “always wrong”.
He writes: “Moreover, recent research from the National Marriage Project indicates that infidelity has also declined in recent years to the point where just 16 percent of married men and 10 percent of married women now report that they have been unfaithful.
“So, clearly, in contemporary America that vast majority of couples reject infidelity in theory and practice.”
Wilcox continues: “When it comes to marriage, one of the few bright spots to emerge over the last forty years is increasing public support for sexual fidelity—in both theory and practice. Indeed, social science tells us that married couples who remain faithful to one another enjoy higher-quality marriages, lower rates of divorce, and, yes, higher levels of emotional satisfaction with their sex life.
“Sexual fidelity also increases the odds that children are born into and reared in a stable, two-parent home.”