When a fundamental institution of human life undergoes major changes, there are probably more than one factor at work. There have been several interesting articles recently that consider different aspects of this change: the capstone model, the trust deficit and the socio-economic factors that create "unmarriageable men."
W. Bradford Wilcox notes that the "highly educated adults have embraced a “capstone”
model of marriage that typically leads them to put off marriage until
they have had a chance to establish themselves professionally,
personally, and relationship-wise." She argue that they reap many advantages, especially financial ones. ( http://www.slate.com/articles/double_x/doublex/2013/03/marry_in_your_twenties.single.html).
While they may be waiting to have children, they aren't waiting to live with a man. However there is ample evidence that cohabitation does not foster positive marriages when they do enter into them (http://blog.heritage.org/2013/03/14/family-fact-of-the-week-the-feminine-mystique-and-feminists-mistake/). In addition the lives created by these "professionally successful" women have often left them feeling less satisfied with life despite Betty Friedan's promises. As Danielle Crittendan grants in her book What Our Mothers Didn’t Tell Us: Why Happiness Eludes the Modern Woman, “For
all the ripping down of barriers that has taken place over a
generation, we may have inadvertently also smashed the foundations
necessary for our happiness…American women have achieved the most
egalitarian marriages in the history of the world. And yet they actually
feel more oppressed in their marriages than their grandmothers did. How is this possible?” (cited in http://www.biblicalwoman.org/voices/index.php/2013/03/05/why-are-women-still-unhappy/).
Karen Swallow Prior has written an excellent piece in which she points out the negative outcomes that a capstone model produces. She describes the strengths of a cornerstone model in marriage (http://www.theatlantic.com/sexes/archive/2013/03/the-case-for-getting-married-young/274293/).
Unwed women having children in their teens and twenties has become far more common among lower socio-economic groups. Wilcox calls attention to the trust deficit created by divorce and argues that this has hit these groups harder (http://www.slate.com/articles/double_x/doublex/2013/03/marry_in_your_twenties.single.html).
Derek Thompson examines the economic side of this problem. Because of economic changes, there are fewer marriageable men in lower socio-economic groups who have something to offer a woman. Women are instead choosing to remain unmarried, even as they drift in an out of relationships and have children in the process. The results harm the children and beginss a self-sustaining downward spiral. (http://www.theatlantic.com/business/archive/2013/03/the-decline-of-marriage-and-the-rise-of-unwed-mothers-an-economic-mystery/274111/)