Four out of every five babies born at Parkland Hospital in Dallas do not have a father listed on the birth certificate.
The problem is far more involved than just when people get married.
One issue that I've seen, often, is that many of these teenage girls are impregnated by men who are much older (5+ years) and are the victims of what is ostensibly statutory rape. That's probably a significant contributor to not listing the name of the father. The other issue is that these girls tend to come from homes with low educational attainment, with high levels of conflict and dysfunction, in areas of high poverty. So, we are talking about a specific subsection of society.
From Wikipedia, but this research is very consistent across the board:
[edit] Age discrepancy in relationships
According to the Family Research Council, studies in the US indicate that
age discrepancy between the teenage girls and the men who impregnate them is an important contributing factor. Teenage girls in relationships with older boys, and in particular with adult men, are more likely to become pregnant than teenage girls in relationships with boys their own age. They are also more likely to carry the baby to term rather than have an abortion.
A review of California's 1990 vital statistics found that men older than high school age fathered 77 percent of all births to high school-aged girls (ages 16–18), and 51 percent of births to junior high school-aged girls (15 and younger).
Men over age 25 fathered twice as many children of teenage mothers than boys under age 18, and men over age 20 fathered five times as many children of junior high school-aged girls as did junior high school-aged boys. A 1992 Washington state study of 535 adolescent mothers found that
62 percent of the mothers had a history of being raped or sexual molested by men whose ages averaged 27 years. This study found that, compared with nonabused mothers, abused adolescent mothers initiated sex earlier, had sex with much older partners, and engaged in riskier, more frequent, and promiscuous sex.
Studies by the Population Reference Bureau and the National Center for Health Statistics found that about two-thirds of children born to teenage girls in the United States are fathered by adult men age 20 or older.[31]
[ame=http://en.wikipedia.org/wiki/Teenage_pregnancy]Teenage pregnancy - Wikipedia, the free encyclopedia[/ame]
HUGE ISSUE, and we never talk about it.