The heroine of this story is an English Quaker whose main principle in life is complete openness and honesty all the time, even in situations where a "white lie", or just keeping one's mouth shut, could be used to smooth out a social situation. I.e. she is painfully naive and stupid. The hero is an unreconstructed rake who thinks nothing of offering to take up this naive innocent as his mistress because he's attracted to her, and thinks that a wife's main principle in life is to be obedient to her husband in all things. The book is about how these two extremely unlikely lovers come together as a couple and IMHO it doesn't work. Even three-quarters of the way through the book, and after they had been married for a month, the two still seemed like strangers to one another.
This theme (of the Quaker and the rake) was employed much, much better by Laura Kinsale in her "Flowers from the Storm".