I hate to admit, but the good Marquis is a man after my own heart. Condescending, supercilious, yet he has a way with words. The translator, Coward, is clearly a fan - and - to be sure, the Marquis's reputation is built largely upon a few sexual peccadillos when it should rather be built upon his philosophy.
That is to say, much, if not all he did was a rebellion against the "Virtues" Church and State imposed upon it's involuntary members, yet rarely practiced themselves.
His irony and contempt of the mores expected of him merely reflect the hypocrisies recommended the populace by it's leaders.
So, in a sense, still very relevant today. His primary relevance lies in speaking truth to power.
And he does so in so very droll a fashion, the descriptions, for example, by Justine of her rape, told by the victim as if it were the sole intent to inspire and titillate the reader.
He was not unaware.
Anyways, a good deal more intelligence than you'd suspect, given his reputation, and - very much like Celine and Henry Miller he was very much out to provoke his audience. And he succeeded.