] That always struck me as a mistake, because it seemed to me that what was "acceptable" for members of the aristocracy was quite different from what the gentry considered acceptable for themselves.

Quite true. I didn't realize it before I read that book but it makes a lot of sense to me and explains why a Cholderlos de Laclos could depict such a depraved upper class where evryone has just tons of extramarital affairs and JA described such a tame one where such things seldom happened-but the people JA described are genteel, not aristrocratic (and I always thought that this is the same!). I simply threw upper class and aristrocracy in one box. But it's very important to distinguish the two. I also think that a lot of the marriages à là mode took only place in the aristrocracy where marriages still were arranged.