As a historian of private life, this is my area of expertise and yet I find this question very difficult to answer. Instead of an authoritative answer, let me offer a few impressionistic examples:

• The biographies of quite many intellectuals in twentieth-century Europe involves extramarital affairs, even non-sexual ones that are developed over correspondence. Sartre and de Beauvoir had multiple affairs, and biographers still debate how willfully de Beauvoir agreed to her husband's womanizing tendencies. Kafka was thoroughly in love with Milena Jesenská, even though they met only a handful of times.
• Two surveys from the 1920s Soviet Union, a period of relative sexual freedom, demonstrate a rate that can be interpreted as rather high or rather low of women committing -- or, more precisely, admitting to -- extramarital affairs. In 1927 Odessa, 6 out of 142 married answered affirmatively, with 3 of them claiming to have had three or more affairs. In Omsk, the rate was 5 out of 50. The respondents were students. For Omsk, see Kliachin, Половая анкета среди Омского студенчества, Sotsial'naia Gigiena 6 (1925); for Odessa, Lass, Современное студенчество (1928).
• As Michiko Suzuki demonstrates in an article called "The Husband’s Chastity: Progress, Equality, and Difference in 1930s Japan," even though the Meiji Code of 1898 made monogamy the legal norm, it was still common to hold a mistress or have some other kind of extramarital relations.
• Alfred Kinsey et al. published two books on human sexual behavior in the late 40s and early 50s. They based their data on surveys including 5,300 males and interviews with 6,000 women. While the British public readily accepted the fact that Americans are sexually promiscuous, they were shocked to find that this "repugnant" behavior was endemic to their country too. One-third of the respondents were willing to "give approval to some form of sexual relationship outside marriage", which was basically meant cohabitating or engaged couples, but all the more shocking to the public was to hear that a quarter or husbands and a fifth of wives admitted to being unfaithful.
• In some rural environments, extramarital affairs were sometimes rooted in folklore. The head of the household in many Russian peasant households was known to have "daughter-in-law privileges", snokhachestvo in Russian, which essentially meant being allowed to have sex with her while his son is either absent or is still minor. This was particularly common in places where the age of marriage for males was significantly lower than that of females.