I don’t think there’s been a situation of over 40% illegitimacy in any society with the concept of “marriage,” which is every society we know about.
That's actually a really good question, so I went to look it up, see if there was ever any research done on that. It appears there was
a research article written on the subject in 1972 that compared rates. It says in the summary that "illegitimacy rates in European populations after 1750 finds a period of rising rates to around 1870 followed by one of declining rates to about 1940. The early rise in illegitamcy appears to have been related to increasing sexual activity, while the decline is ascribed to increasing use of birth control." The rest of it is behind a paywall, though, but the citations are available. Might be an interesting subject.