It's designed to be for adult entertainment sites as a way to help parents and others who want to stay away recognize them and for those who visit them to know they're safe and operating above-board.
Red staters tend to visit sites devoted to adult webcams and voyeurism, for instance, while blue staters prefer sites focused on professional adult entertainers and escorts.
That said, there is a silver lining: as Network World goes on to explain, these days you are actually less likely to get malware from "adult content" sites than in previous years.