There are two or three reasons for this. Yes, the “Edwardian” period following the Victorian one, was a time of greatly relaxed standards regarding fidelity, particularly in the upper classes in Britain, as had e.g. the “Restoration” period following the Roundhead or Puritian ascendencies in the late 17th century.

More generally, in British and French aristocratic circles where arranged marriages were the almost invariable rule until recent times, extra maarital affairs for BOTH spouses were much more accepted than now, especially when the wife had equal or nearly equal social status in her own right and particularly when she brought in much or most of the money – as was the case with Lady Randolph Churchill.

Still, Jennie Churchill took it very far for someone at her level.

Finally though I’d like to note that there is a strong element of acceptance of cuckoldry by some elements of British society, especially upper class class society. It may not be unrelated to the rather large amounts of homosexuality or bisexuality there as well. Male esthetes and all. In Britain this was not and is not thought to be an exclusively gay thing, and has at various times and places been associated with the liesure class, and high status and “refinement”. I think it also has something to do with the code of male chivalry, which has nowhere been more pronounced than in England. The code involves an ethic of one degree or another of male self sacrifice for the beautiful and perhaps unattainable or only partly attainable woman, who is worshiped and never really fully possessed. This can lead rather easily towards allowing this “goddess”, who one is not really fully worthy of, to have extraordinary and even one sided freedom. [I think this is also a good part of the reason why feminism has gone further in the Anglosphere than anywhere else, including other European countries.]

When that sort of male attitude, or it’s more extreme form, is married to a great beauty and unconventional thinker, in an era of greatly loosened sexual morality lead by the example of the Monarch himself, King Edward (one of Jeannie’s lovers), one can get a Jeannie Churchill.

So – what I’m suggesting is that an essential enabling (but not alone sufficient) reason for the phenomenon of Lady Jeannie Churchill was that her first husband (Lord Randolph Churchill who as a younger son of the Duke would not become Duke and whose son (Sir Winston Churchill) as twice removed would not become even an honorary Lord), was a permissive and probably willing cuckold (male sexual submissive), who very likely encouraged her sexual activities and got his own sexual charge from them.

So ladies if you want to follow in Jennie’s footsteps 1) be really beautiful and witty to boot; and 2) more importantly, marry a socially prominent willing cuckold. They’re a lot harder to find in America, but are by no means non existent (particularly I think in gen X and esp. Y’s with their often rather thorough feminist indoctrination).

Probably greater than the number of men could do the opposite.