They used girls 15-19 in their statistics as unmarried.
How far back are they looking for comparison? Not long ago in our nation's history, 15-19 year olds were the prime demographic to be married. If we're looking at 51% of women as being an unmarried anomaly, then is that taking that historical fact into account?
In other words, if you're going to compare long term trends, you need to keep a stable set of factors to compare it to.
Medved used 2005 census numbers.
http://www.townhall.com/columnists/MichaelMedved/2007/01/18/journalistic_malpractice_in_marriage_is_dead_report