You can found a lot of BDSM story at Nifty.
Their stories are classed by genre.
But it's not for faint heart and it's gay guys that write about gay guys that do the nasty.
Some story can be really hard (institutionalized slavery, uro, scato, torture) but they are easy to see, on top of the story this guys use tags to help the readers.
Story of tim and Child of Mythra are good story to read here.