As much as you might think you are protected from "porn" sites, if you leave your "cookies" file wide open, all sorts of garbage gets fed to your machine.
My sister-in-law, who is probably the person least likely to get on a porn site (but unfortunately is a bit PC-challenged), somehow innocently clicked on a popup that loaded an executable on her PC that would launch on bootup. A nice porno wallpaper for her and her deacon husband to see every night.
I talked her through the process of getting rid of it, and fortunately, it was a harmless .exe file with no hidden malicious intent.
But the point is, unless you are armed with some form of watchdog software, your purest surfing intentions will not shield you from the solicitous garbage that pounds your firewall at nanosecond intervals, especially pornographic ones!