Modern premarital sex rates have never really changed significantly since the 1940's...it is just more openly discussed today.
Before then premarital sex was not as common because people would get married very young or after just a few short months of dating. The long 1.5-2 years dating ritual we have today is a a modern trend.
BTW, a 1/3 of babies were born out of wedlock in early 1700's New England from records kept of the day (fact)
I'm referring to sex in a committed relationship leading to marriage. Yes, one night stand promiscuous style sex is way up.
I believe that statistic is conceived out of wedlock; but not born out of wedlock. Fathers and brothers saw to that. (Did my graduate work on the social institution of marriage in the U.S.)