This brief report, based on the Census Bureau's Current Population Survey, 2014 Annual Social and Economic Supplement, breaks down many other categories of living arrangements, as well, and points out that 86% of college-educated parents are married to each other and living together.
It also points out that it's "only a snapshot at a single point in time" of all families with minor children, and so does not "show us changes in arrangements and relationships that occur as children grow and develop: for instance, if current trends hold steady, by the time they reach 17 years old, less than half of today’s youth will have spent their entire childhood in a two-parent family."
These numbers also show the hazards of unwed cohabitation for children. People, even pro-family researchers, often feel obligated to point out that parents CAN be just as permanent and committed, with equally healthy romantic and parenting relationships, even if they do not choose to marry. Well, they theoretically CAN, but this shows that not surprisingly, very few DO, or at least not for long.