When we think about the issues of family, sex, and morality there's a standard story that frames our thinking. Ross Douthat, writing back in January, did a good job of telling that story:
"[Liberals might want to] acknowledge the ways in which liberalism itself has undercut the two-parent family — through the liberal-dominated culture industry's permissive, reductive attitudes toward sex, and through the 1970s-era revolution in divorce and abortion law."
It seems obvious that sexual permissiveness discourages marriage. In the old days, marital sex was the only socially acceptable kind of sex, so if you wanted to have sex, you had to get married; allow people to have sex outside of marriage with no social sanction, and you take away a big part of the impetus to get married. Liberal values, therefore, are corrosive to families.