I'm actually for this. I can see the problems, and of course there are problems. There always are. This is no reason to keep those of us who would handle this kind of stuff in a responsible manner from enjoying a good, mature RP. Tak, for example, an older and not ver successful RP, "Fallen Angels." I love that kind of stuff, and I daresay that a good few other members would as well. However there, I had to constrain myself and couldn't let my vampiric side run wild. I'd say make it a subforum of the T-Dome.
As for the flood of "mature" RP's that Abby mentioned, I doubt that the people who are actually serious about these things would participate, so the aforementioned flood would in all likelihood, whither and die.
As for flames, yes, RPing can engender such behavior. By the people that RPrs tend not to like. We prefer to have a nice, uninterrupted RP between responsible members that understand that a character may be antisocial and that's why that told you to "get out of my face before I slit your throat." So the flamers probably wouldn't last long there, just as they don't in many of the current RPs. As for the more serious RPrs, those arguements tend to be over little technicalities that we blow out of proportion. I don't know about the rest of PC's serious RPrs, but I enjoy a good arguement like that every now and then. But I also like to constrain it to a PM conversation.
About sex themed RPs, I most certainly wouldn't participate. I doubt that many of the people campaigning for this would either. I don't see this as much of an issue.
For a rating system, there's the obvious problem of people ignoring the ratings. Furthermore, there might be somewhat of a stigma associated with participating in say an R-rated RP that could carry over to other, more benign RPs. With a seprate, request-only forum, those hassles are mostly eliminated.
Finally, FT, I'll take you own for the title of "Master of Bloodshed" any day.