Violent crime in this country, sexual assaults included, has been gradually going down over the last couple of decades. IIRC, we're down to a mid 1960's level of crime these days.
If anything, one could make the argument that the legalization of porn has helped to reduce the crime rate.
People would much rather hear anecdotal musings from a guy on the street than real statistics. Its the same reason why the 55 mph speed limit has stuck around so long.