Easy, but wrong -- we're all to blame, in some sense. This is the confluence of at least three cultural failures.
First, we have come to accept illegitimacy as "normal," to the point where we think it's unfortunate and sub-optimal, but not really wrong. So there's that part.
Second, we've come to accept "living together" as "normal," and in most cases it's not even frowned upon -- it's just a lifestyle choice, among many others.
Third is, of course, sex. Sex is seen as something to be done for its own sake, because it's "fulfilling." As a rule, we don't frown on sex outside of marriage -- the old "consenting adults" thing is supposed to have superceded such stodgy thinking.
And so now we see the confluence of them here, and in too many similar cases. Mother of illegitimate kids once again mistakes sex for love, and shacks up with the man who ends up hurting or killing her kid.
It used to be that marriage was a requirement for cohabitation, sex, and kids -- and that's no longer the case. The culprits are not just liberal Democrats: we've all allowed those conditions to fester and flourish.