I agree that the Achilles Desjardins torture porn was disturbing, but I've always interpreted it as an argument against Utilitarianism, and not BSDM porn per se. It's a more vividly disturbing version of The Ones That Walk Away From Omelas.

Watts sets up Desjardins as a Utilitarian demon, an evil that we tolerate because his specific evils are less than the overall good he provides the world.

In the end it turns out that the overall good he provided the world was a deception, and the man was revealed to have been pure villain for the entirety of books 3 and 4.

But yeah, I agree with you that I'd prefer to have not read those scenes. I'll still recommend Starfish, but not its sequels.