Throughout the story, de Sade constantly tries to “excuse” the characters’ actions, for lack of a better word.
He tells the reader that virtue and vice are social constructs and that Nature made the libertines the way that they are, and if she thought it wrong then why would they be like that?
He also tells the reader that what they are about to read may be the most vile thing they will ever read and that they might be disgusted, but to keep in mind that to others it would… basically turn them on, there’s no good way to say this.
As a side note de Sade also avoids writing out saying some of the punishments the four main characters perform, saying that nothing he would say would satisfy the reader (presumably because he thinks what he has in store for later will). He does something similar to this later on, saying that he could not possibly describe the “obscenities and abominations” of Durcet’s post-dinner activities.
At a different point, Durcet (oh look it’s him again) puts down Zelmire’s name for punishment, as the four protagonists wanted to put all eight girls down for punishment, and at this point, it was only six. Now, de Sade says, whether or not she actually did anything wrong, whether she deserved it, is for Durcet to know, and the reader must not question, and to just be content with the story being told.
In all honesty, I could’ve based my reflection around other aspects of the story, like how in the first bit he refers to the four protagonists both as heroes and villains and how he carries that paradoxical narrative throughout, but I didn’t actually want to think about the worse parts of the book for too long, so I went for this instead.