What you described was somewhat of the standard in 1800s and pre-1960 United States. The 1960s is when there was a loosening of sexual cultural standards in the United States. The introduction of the birth control pill in 1960 and the adoption of no-fault divorce in most US states in the 1970s are 2 key events in the loosening. California was the first state to pass a no-fault divorce law in 1969. This 2009 article is a good history of divorce in the USA from 1960s-2000s.
https://www.nationalaffairs.com/publications/detail/the-evolution-of-divorce
The loosening of sexual cultural standards has been associated with pre-marital female promiscuity.
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A lot more men are getting a woman now with a substantially higher notch count than in the 1970s. Also, the 2010s data points end around 2012-2013. Imagine what it is like now in the 2020s.
A lot of men figure out how to use elements of Game (looks, money, status, and personality/charisma tactics) between 25-34. A more Game-aware guy in his late 20s might be using psychological seduction tactics that women were using on him 10 years earlier. If a 29 year old guy is using seduction tactics on a 26-27 year old woman, he's using them on a woman who has had 10 years to put up notches.
This is a common scenario for men in their 30s/early 40s. The more Game-aware guys will use this knowledge and focus on either casual sex or serial monogamy without legal commitments. There are still enough single male thirsty betas out there who outnumber single females who will commit to a woman with a high notch count.