It's obvious that de Merteuil is the smartest and most capable person in the whole thing, all the way until the end, and the hypocrisy and double standards about infidelity committed by men or women are highlighted. On the other hand, some characters also explain and justify said double standards, and due to the ending, you certainly could interpret the novel as agreeing with that stance, as well as condemning de Merteuil. Like often with this kind of older novels that have endings which irritate and confuse modern readers (Oliver Twist, The Mill on the Floss, etc.), the interpretation making most sense for us is to assume that the author intentionally spoiled their own novel with an awkward, not very credible "happy" ending, kowtowing to the pressures of public opinion.

Seems like a fair middle road in terms of interpretation.