Via erosblog: According to Mark Kleiman (and the Department of Justice), forcible rape has gone WAY down in the United States over the past few decades. It peaked in 1979 at 2.8 per 1000 population 12 and over, and in 2009 the rate was .5.
Like homicide, the rate has really tanked since the 1990s, but the downward trend for rape has been more secular and more dramatic than that for homicide.
As Kleiman notes, it does seem like pretty damning evidence for the theory that pornography promotes rape, as a practical matter. And there's some evidence that access to porn prevents rape.
Culture is not my favorite explanation for human behavior, but I think there might be something to it here. Rape used to be accepted as a natural consequence of sluttiness (even the species of sluttiness where you merely let yourself be alone with a guy in a room); see, e.g., that gem of criminology writing from 1967, "Victim-Precipitated Forcible Rape" by Menachim Amir. Pornography was forbidden for the same reason: to repress sexuality.
It's now extremely clear that sexually permissive societies are better for women. Only the very religious and your dad believe otherwise.
Also: here's a 2008 paper with up-to-date information on what we know about the evolutionary biology of rape, which I found while doing my sacred duty to humanity.