MotM has not strictly had an official rating for some time. Historically, much of the material concerning what was or was not allowable was silly and melodramatic. More or less, MotM now supports a simpler ruling: On MotM, you are assumed to be an effective and responsible adult, with all of the reasonable faculties thereof.
MotM is bounded by what's considered generally accepted good taste, which means that your conduct--both public and private--is expected to conform to a reasonable amount of common sense. In the new world of MotM, the feature characters might be sex demons or have torn out people's innards in their source materials, but our hearts and our stomachs tell us that pursuing these angles is not the right direction to go in your standard garden variety roleplay scene. Be reasonable, and keep overtly sexual themes and extreme hyperviolence in private with the express consent of everyone involved in the scene.
General hot-button character concepts, such as extreme and specific real world racism, misogynism, or general bigotry are also considered to be outside the bounds of taste on the game. The same can be said for acute non-reversible dismemberment of non-consenting or feature characters.
It's worth noting that while nobody is going to jump out of a bush and beat you with billyclubs for letting loose the F-bomb in the public room, staff fiat is pretty much in effect for all judgments of good taste, no matter where it occurs. If your character suddenly grows a third penis or you are found to be using the combat system as a marital aid, you will likely be politely asked to stop. ;)
This is one of those things that typically end up being hotly contested and argued with a litany of excuses and examples, but nobody is really interested in having an argument for three hours about the appropriateness of a sixth nipple. Definitions of appropriate vary wildly. Because of this, the judgment on what is and is not appropriate is ultimately up to staff, and language concerning 'PG-13' is specifically not bandied about for that reason. This gives people the freedom to speak freely without having to worry about someone counting how many times they've said the word nipple, but at the same time allows the more culturally sensitive amongst us an avenue of respite. It's important to remain aware of everyone around us when we're talking.
For this reason, it's best to do your evil deeds responsibly, where the Man can't catch you and throw you into deep orbit. ;)