" Norms and practices have clearly moved in the direction of increased tolerance for promiscuity over the last century, though of course they aren’t remotely near an extreme free love scenario."
I'm not sure what country and class the good professor has in mind, but he may be misled by popular history. For example, my understanding is the British upper classes, as represented say by the Victorians (think of Churchill's parents), were very promiscuous; once you had an heir and a spare in the bank, wives were home free whereas the husbands were always free.
Elsewhere prostitution was more prominent in urban areas in the 19th and first half of the 20th century. And notoriously, one-third or more of the first children of the Puritans of New England were born "prematurely".