One of the common themes we hear in the Mormon church with resepect to sexual morality is that societal standards have deteriorated, and that we should return to those more virtuous days when sexual morality was the norm.
Let's be clear about something though. Those virtuous days are a myth. They never existed.
When I was in graduate school, I read a report from the Guttmacher Institute1 that claimed over 95% of Americans had sex prior to getting married. What's more, that trend has been in place since before the 1950's. I didn't believe the report was accurate, so I replicated the study using CDC data from the National Survey on Family Growth. Much to my surprise, I found very similar results.
Premarital sex is the norm in the United States (and the world), and it has been for much longer than you think.
I recently listened to a podcast from Mormon Matters in which one of the participants described a article that reviewed births in the 19th century with a focus on births that happened less than 9 months after the wedding. Unforutnately, they neither used specific numbers nor cited a reference that I can find, but I'm trying to verify the claim.
The evidence I have gathered so far indicate that 40% of women in the mid-19th century were giving birth less than 8 1/2 months after marriage2. Certainly, some of these were preterm births, but if you add in miscarriages and women who were having sex with the good fortune of not getting pregnant before their marriage, I would imagine that percentage is still higher than 40%.
Another, less scientific, evidence I came across was in a book Tracing Your Family History. The author advises people not to look for marriage records only prior to the birth of an ancestor, but also after. He says:
Marriage records are often missed because people assume their ancestors practised the rule of 'no sex before marriage.' After many years as a full-time genealogist I have serious doubst as to whether many people have ever heeded that maxim...So, anyway, I really doubt the past was any more virtuous than the present. At least if by 'virtuous' you mean the distorted definition of the word that is in use by the Church (and rightfully, a definition that is currently receiving much criticism). It would be more accurate to say that I really doubt the past was any more chaste than the present. (I have no doubt that in many ways, the present is more virtuous than the past3).
If you're going to say that anything has changed between the past and the present, you are probably limited to saying that we talk about it more now than we have in the past. There are good things about that, and there are bad things about that. But when we have discussions about sexual morality, let's not fool ourselves into thinking that our ancestors were so pure. They weren't.
1 Summary report and detailed report
2 MINTZ, Steven, and Susan Kellogg. Domestic Revolutions: A Social History of American Family Life. NY: Free Press, 1988. (summary here)
3 For instance, women who were raped in the past were stigmatized, considered damaged and impure. Such a vile stigma is the antithesis of virtuous.