Speaking of incest, in FF before the family is found by the black rulers I think there was a conversation about repopulating the world where the daughter seems quite willing to have a child with her father or with Joseph (the black houseboy/accounting student). I think that Heinlein often extrapolated the fears of American society that were current when he wrote. For example, FF was published during the civil rights era and the cold war so it dealt with nuclear war and also inverted the relationship between the races, placing the blacks in a position of power over whites. Another example might be his treatment of female characters such as Friday, Deety, and Maureen following the feminist movement.