The Idiot Steaminess Rating
All the sex that the novel mentions has either already happened or will happen sometime in the future, so for all the frank talk about mistresses and passionate feelings, there aren't actually any descriptions of anyone doing anything naughty with anyone else. (Nor with themselves for that matter.)
Still, for a 19th century novel, this does get pretty graphic in terms of laying out the fact that General Ivolgin is screwing around on his wife, that Nastasya was forced to sleep with Totsky without being married to him, that Ganya is grossed out by the idea of Totsky's sloppy seconds, and that it's really weird that Myshkin lived with Nastasya without trying to get in her pants.
The only potentially dirty part of the novel is never really addressed by anyone—but we'll point it out, cause that's just the kind of minds-in-the-gutter readers we really are. Do you think Rogozhin and Nastasya had sex before he killed her? After?