There seems to be some inconsistency in this discussion. The "married, two parent household" seems to solely focus on heterosexual marriages, and "unwed mothers" seems to be used to indicate absent fathers, as opposed to couples in committed relationships who are simply not married.
The last type is becoming more and more popular, as the traditional uses of marriage (political alliances, transfer of women from family to family) are becoming obsolete in the face of social and economic equality for woman, and the economic uses of marriage (inheritance and joint property) no longer require a marriage to function.
The OP is talking about a rise in unmarried women having children. It doesn't differentiate well between single mothers and mothers in relationships. The idea of permanent linking just seems more dangerous now than it used to. It is completely possible to go through life unmarried. That was not the case a few centuries ago, when women had to marry for economic stability. Now, with the uncoupling of sex (and thus children) from marriage, marriage is no longer as needed.
Ms. Strader, in the article, is unwilling to risk marriage, as the men in her life weren't up to snuff. But should that limit her from having sex and raising her children? She doesn't need to marry these men, and doesn't need to take the risk.
Basically, legal mingling of two people is no longer the advantage, and certainly not the necessity, it once was.