In my opinion, one of the greatest challenges a serious writer should take on is to make the "ordinary" interesting . Stretching reality (and I do it too) is often the easy way out.it is very difficult to put together a credible story without stretching reality at times - i haveto agree that soemtimes the suggested tortures or sexual actions would be so damaging or impossible physically that the plot sort of becomes too far-fetched and loses the reader.
Read the opening to Dickens "A Tale of Two Cities" as an example. Isaak Denison's "I had a farm in Africa" (Out of Africa) is another. Ordinary circumstance rendered into a magnificent image.
A great artist - van Gogh, Rembrandt, etc - takes an ordinary scene and renders it into a memorable painting. Great writing does the same thing.
If the subject is fantasy - Hieronymous Bosch or Harlan Ellison - the same applies. Make the fantastic believable, make it real, make it credible. Shakespeare's ghosts are real characters to the audience because of the skill of the writing, not because ghosts exist.
With respect to BDSM stories, I agree that in much of the genre the fantasies are far-fetched, often too much so for my personal taste. But that is highly subjective - for example, I don't read Harlequin Romances or pulp science fiction but millions do.
I write to express my ideas and creativity, I work to provide entertainment for others. I believe it is incumbent upon the writer to know who will be reading the piece, and to deliver the plot in such a way as to captivate the reader.
Final comment: "It is very difficult to put together a credible story ..."
Of course it is. Writing is a creative process, it involves hard work, perserverance, and effort. I do not know of a single professional writer who does not work at it on a daily basis.
Striving for excellence requires a vision of something better and the determination to make it happen.