In "Book Three: The Judges" of The Poisonwood Bible, why are Orleanna and Nathan fighting?

About 80 pages into the section, in a chapter narrated by
Ruth May, Orleanna and Nathan fight over the fact that some of the tribal elders are
saying that Rachel needs to be circumcised so that she won't be promiscuous later on in
her life,  "so she wouldn't want to run around with people's husbands."  This highly
controversial and brutal act of circumsizing females is often practiced in Africa, as a
way to stem the spread of AIDS, children out of wedlock, and promote male dominance. 
Nathan and Orleanna agree that it shouldn't be done, but Nathan ties it back to his
missionary work, and how he needs to teach the African people other ways so that they
don't lead "these female children like lambs to the slaughter."  Orleanna, at this
point, gets upset and accuses him of caring more for the African daughters than his own;
Mathan at this point is playing along with the tribal chief's request to have Rachel for
his wife.  Orleanna also doesn't like Axelroot hanging around Rachel and implores Nathan
to make him leave, but he concludes that Axelroot is better than the natives, and much
more safe.  Orleanna is so upset that she tears a sheet in
two.

That is the most specific instance of fighting that I
can find.  Throughout the entire book is the thread of Orleanna hating it there, being
worried about Ruth May's illness, and declaring outright that she wants to leave. She
stops supporting Nathan in his insane perspectives, and becomes a bit more independent,
which causes strife in the marriage.  I hope that those thoughts helped; good
luck!