The Chrysalids/Fridge

Given the fact that David and Petra are exposed as mutants, their mother would have had four mutant children, thus it's very likely that Joseph would have banished Emily to the Fringes as part of standard Waknuk law.
Sure, Gordon tried to rape Rosalind, but keep in mind, he said that both Rosalind and Petra (his own niece) were fertile. What part of the book says that he wouldn't try and "have relations" with Petra?