I have a theory about this.
I'm forever hearing people moaning that they have received porn over the net, totally unsolicited of course! I reckon it's just the cookie system at work - in other words, if people have porno related cookies on their hard drive, other porno sites will be connected to them through their cookies.
So what is probably happening is that the people who complain most about getting unsolicited porn are causing it themselves by having visited porn sites in the first place - ahaa, gotcha!!