The advent of birth control itself can't be the main cause. I think the biggest problem is that the social stigma associated with having children out of wedlock eroded over time. It used to be that women who did that were social pariahs, but not anymore. Now they don't get married just so they can rake in the welfare benefits.
No-fault divorces also surely helped in that marriage can so easily be dissolved, fostering the attitude that marriage isn't sacred anymore. And all the children bearing the burden of that grow up in single-parent homes, and I believe just about every study done on the subject shows that such homes are not as good as two-parent homes for raising moral, well-adjusted kids (the kind who are less likely to sleep around a lot).
Looking back on that text, it seems to be a vicious feedback loop.