Alright, design principles of a pornoverse:
First, obviously, lots of sex happens. Most people want to be having sex just about as often as they're physically able to, and even those who don't are willing to play along.
This attitude, which would likely be considered addictive behavior in our world, is regarded as normal; nobody gets traumatized, the worst reaction is embarrassment or annoyance. People have interests and pursue activities other than (though seldom entirely unrelated to) sex.
My thinking with defining the mechanics first and the setting afterward was that, in a pornoverse, most social conflicts are rooted in the problem of how everyone involved is going to satisfy their respective libidos, the same way an agrarian society's conflicts revolve around who does the work and who eats the food.
The rules for orgies create a sort of Prisoner's Dilemma situation: cooperate, roll at a penalty, and help everyone get off? Or defect, and maybe benefit from everyone else's hard work? Bonus for defecting goes up the longer you've gone without, but then being greedy risks alienating a new partner, which could be a vicious cycle, until someone saintly enough (or more likely, who has a kink you perfectly satisfy) comes along.
As for the grisly details... I didn't spell out any mechanical differences between fisting vs. penis vs. tentacle, or oral vs. vaginal vs. anal, or sundry other limbs and orifices, natural or otherwise. There's nothing about specific positions, except that they can be defined later and then bought up as techniques to simplify group sex with strangers. There's only one new skill, which is an extrapolation of an existing set of skills. No lurid descriptions of new overpriced Advantages.