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The needle on America's moral compass is on the move, although not when it comes to extramarital affairs.

American standards for morally acceptable behavior have shifted dramatically over the past 16 years, as growing numbers of adults espouse more liberal views on gay and lesbian relations, sex out of wedlock and divorce, according to new research from Gallup.

This "leftward movement in perceptions" affected more than two-thirds of the controversial practices and behaviors explored in the survey, which ranged from stem cell research to watching pornography. But one old-school sin was almost untouched by the trend.

Gallup poll results. | Aaron Thorup, Gallup

Extramarital affairs have nearly the same moral status today as they did at the turn of the 21st century, with 9 percent of adults calling them morally acceptable in 2017, compared with 7 percent in 2001.

"Whatever Americans may think about the Ten Commandments as a whole, they're on board with the seventh commandment: 'Do not commit adultery,'" noted Facts & Trends magazine in its coverage of Gallup's research. Adultery had the lowest moral acceptability rating in the study.

Americans' relatively stable opinion of extramarital affairs is notable because views on sexual relationships are generally the most malleable, according to the survey.

"Since the early 2000s, the percentage saying that gay or lesbian relations, having a baby outside of marriage, sex between an unmarried man and woman, and divorce are morally acceptable have increased by double digits," Gallup reported.

Although a vast majority of Americans view adultery as morally wrong, disagreements related to this behavior spring up in other ways, as the Deseret News reported last month.

Digital technologies, such as smartphones and social media sites, influence people's understanding of infidelity, creating a gray area for couples.

Deseret News/Y2 poll results. | Aaron Thorup, Deseret News/Y2 poll

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Half of Americans "say sending flirtatious messages is always cheating, 19 percent say the same about watching pornography without a partner and 16 percent say following an ex on social media is always cheating. But in each of these cases, at least 25 percent of respondents say these activities 'sometimes' constitute cheating," the Deseret News reported, drawing on a survey commissioned for the article.

Couples should be aware of the potential for partners to define cheating differently and be open about their expectations, said Katherine Hertlein, a therapist and professor at UNLV.

"I'm very surprised when I ask a couple about their definition of infidelity during premarital counseling and they respond, 'What are you talking about? It means not physically touching someone else,'" she said. "I tell them they need to wake up. It's 2017!"