My Mom was ill and staying at my Sister’s home. About a week before she died, she gave a book to my sister, saying that it was the secret to a happy marriage. My sister was not really interested in the book, so I took it to see what the secret was. The book is “Natural Birth Control — In Harmony With Catholic Morality”, published in 1938. The book is about having sex and controlling whether or not there is potential for a pregnancy. It goes into depth with respect to cycles and the like. It must have worked in this case, because all of us kids were somewhat evenly spread out time wise; and I assumed this was intentional.
But the implied subtext was that much of the secret was that my Dad could be satisfied, and that my Mom was willing to set up a system where this could happen (tracking stuff with charts and the like). And my parents were happily married.