The Pew Research Center recently looked at the percentage of people who are 25 years old or older who have never been married (using the American Community Survey of the U.S. Census). What they found was that the percentage has risen rather dramatically in the last 50 years. In 1960, only 8% of women and 10% of men 25 years old or older had never been married. By 2012, that had risen to 17% of women and 23% of men. That's a very significant jump in percentage.
Why has this happened? A part of it is probably the poor economy. Note that 13% of 18 to 24 year olds, 34% of 25 to 34 year olds, and 29% of those 35 & older said they had not gotten married because they were not financially prepared. But that can't be the biggest reason, because the percentage of "never-marrieds" has been rising steadily since 1960 -- both in bad and good economic times.
I think the bottom of the three charts above probably gives us the best answer. Back in 1960, it was not socially acceptable for a couple to live together without being married. That is no longer true. Our society has changed. Note that a large segment of our population (about half of the general population) no longer sees anything wrong with a couple living together without being married. The group with the highest percentage thinking couples should be married before living together were people 65 or older -- and only 65% of them think so (which is a lower percentage than I would have guessed).
I know that right-wingers (especially evangelicals) really hate to admit it, but we live in a much more liberal country socially than 50 years ago -- and that is a very good thing.
The chart below shows the percentage of each racial or ethnic group making up the "never-marrieds" in the United States (for the general public, and for men and women).