Legalizing Abortion Spread Venereal Diseases: Law professors Jonathan Klick and Thomas Stratmann wrote a paper entitled "The Effect of Abortion Legalization on the Incidence of Sexually Transmitted Diseases." From their abstract:
Using CDC data on the incidence of gonorrhea and syphilis by state, we test the hypothesis that judicial and legislative decisions to legalize abortion lead to an increase in sexually transmitted diseases. We find that gonorrhea and syphilis incidences are significantly and positively correlated with abortion legalization. According to our estimates, abortion legalization might account for as much as one third of the average disease incidence.
Not only did unmarried people have coitus more, but my impression is that the introduction of the oral contraceptive in 1964 and the legalization of abortion in 1970-1973 sent condom usage into an eclipse until the accumulating venereal epidemics (herpes, AIDS, chlamydia, etc.) brought the condom back into fashion in the 1980s or 1990s.
Besides being icky, this STD trend has some implications for the abortion-cut-crime theory. Steven D. Levitt's notion rests on his claim that legalizing abortion increases the "wantedness" of the survivors of legalized abortion. While this sounds plausible at first, illegitimacy rates soared, which suggest that wantedness by the surviving babies' fathers definitely didn't go up.
Klick writes about his finding of a big jump in VD rates stemming from legalization:
This too provides some indirect evidence against the Donohue and Levitt hypothesis, since arguably abortion legalization could lead to an increase in unwanted kids (woman has sex w/o protection because of the security that she could have an abortion, but then cannot go through with it; or women who are against abortion face competition from women who are willing to have abortions, so to compete, they need to be willing to have unprotected sex, etc).
Legalized abortion reduced unmarried women's position in relation to the men who wanted to have unprotected coitus with them. That wasn't good for their children.