Following up on the previous post and its implication that the First World War was the time when the rot set in with regard to Western Civilisation, the following data from the research of Mr Lewis T Terman seems appropriate.
Sex research existed before Kinsey and while limited in scope, seemed to be more objective and of higher quality than nearly all of it that followed it. One of the few researchers in the field at that time, Lewis P Terman, published a book in 1938, The Psychological Factors in Marital Happiness. This was a wide ranging study looking at what made marriages happy and it involved 792 couples. It is of interest because it was one of the first studies to ask about premarital sex. He found the following rates of premarital sex amongst women in the following cohorts.
Before 1890: 13.6%
1890-1899: 23.6% (This is the generation that reached young adulthood about WW1)
Post 1910: 66%
Now admittedly most of the women who did have premarital sex did so with their future fiancee, and sleeping around did not really gain societal traction till the 50's, but what really stands out is the change post WW1. Then as in now, it was the lower classes that lost their virginity first, whilst the better educated and higher classes kept chaste till later. Still the big changes seemed to have happened about the time of the First World War. It really was a different world back then.