Patriarchal societies built rules about women's sexual practices because they wanted to ensure bloodlines and property rights, she says. Until the modern era, the Bible notwithstanding, there was little attention paid to male adultery.
"There was a lot of concern that a woman would be unfaithful and introduce a foreigner into the bloodline," she says. "But infidelity was a practical affair, not an emotional one. It's not until the late 18th century that love becomes a primary reason for marriage as opposed to a lucky side benefit."
Still, over the next 130 years, women were so dependent on men that they simply looked the other way when it came to infidelity, she says. In the 19th century, women were considered "too pure" to enjoy sex; well-bred men went to prostitutes and mistresses to avoid "contaminating" their wives with their urges. Even in the "Mad Men" era of the 1950s and '60s, Coontz says, the discreet businessman affair was often just accepted.
"The men who supported women sort of thought of it as their perk," she says. "And the women who were supported by them took the 'don't ask, don't tell' [approach]. Nobody wanted to be hit over the head with it, nobody wanted their husbands to fall in love with another person, but they shut their eyes."
During the women's liberation movement of the '60s and '70s, the attitude about infidelity shifted. Women demanded that their husbands be as faithful as they were expected to be; or they suggested that they, too, be able to sleep around. About the same time, conservative Christian authors started publishing advice books about sex -- an effort to keep both genders from straying. Still, the divorce rate rose.
The American inclination today to both abhor and accept infidelity is likely a reaction to that time period, Coontz and other scholars say. It is also a testament to our relationship with the institution of marriage itself.
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Americans revere marriage. Historian Nancy Cott, a professor at Harvard University, has written about how our approach to marriage is hard-wired into our DNA. From the earliest days of the US, the Founding Fathers supported the sort of marriage where a benevolent husband was chosen, and then obeyed, by a loyal and loving wife. It was the husband's job to lead and make rational decisions; the wife's, to support him but also act as a check on his unbecoming urges. In some ways, this relationship reflected the political ideals the Founders envisioned for the young nation.