CERO Z is not "specifically for porn" (technically, I don't think ESRB's AO rating is either, but bieng 'murica they are a lot more lenient on violence than sex), it is commonly applied to games that feature extreme violence, criminal behaviour, and such. Some examples of CERO Z-rated games are Gears of War, Grand Theft Auto, Dead Rising, Assassin's Creed, Oneechanbara and Ninja Gaiden. CERO Z games must be sold in a separate "adults-only" section, which isn't quite the same retail death penalty as ESRB AO.
FWIW I'm farily sure CERO does not rate PC games, so pornographic games generally do not carry any kind of rating.