The only study of human sexuality from anywhere close to that time period that I am aware of is the Kinsey study.
Kinsey estimates that "approximately 50% of all married males had some extramarital experience at some time during their married lives" and that "26% of females had had extramarital sex by their forties."
So while it may or may not reach the levels found in a fictional setting, infidelity was something that was not uncommon.