Sexuality and American Culture

I cannot for the life of me recall where, but I once read the following hypothetical ethical situation:
   A brother and sister backpack through Europe.  One night, in their own private room at a hostel, they decide that they want to have sex.  They use careful protection (for the sake of the story it may even have been that one of the siblings was sterile), and they have sex.  The experience is pleasurable for them both and they never do it again.  It remains a private experience and they do not share it with anyone else.  No pregnancy results from the union, and neither sibling regrets the experience. 
The question is: did they do anything ethically or morally wrong?  Is it still a forbidden love?  On one hand, we would like to say that there is nothing wrong with what they did; no one was harmed physically, emotionally or psychologically.  No hairshirted or toadskinned baby is (or could have been) conceived as a result from the union.  But still, somehow, something about the situation is uncomfortable.  What is it?
That brings me to Eugenides’ novel, in which an incestuous union is the clear cause of Cal’s confused sexual identity.  From birth to rebirth (to re-rebirth) our narrator is the victim of the ill-fated (genetically speaking) union of Desdemona and Lefty.  Though they attempted to create a new love story on the boat, they could never leave their genes behind, and eventually their secret manifested itself in something called “5-Alpha-Redutase Pseudohermaphrodism.”  By the end of Book Two we do not know the exact details of Calliope’s journey to become Cal, but we do know the genetic background for it.  As Milton would say, “It’s science, Ma.”  It is this secret science that Desdemona fears for her entire life, and though she cannot identify it as genetics, she is aware of the potential repercussions.
This is where Foucault’s discussions of scientia sexualis and the socialization of procreative behavior come together.   In the nineteenth century, the working class set out to “formulate the uniform truth of sex.  As if it suspected sex of harboring a fundamental secret (69).”  Ironically, in the case of Desdemona and Lefy, it did harbor a secret— their sibling relationship.  Because they loved each other, they could not deny each other the truth of their feelings, and so even though Desdemona swore no children would come of the union (exercising control over procreation), she could not ultimately refuse Lefty.  And later with Milton and Tessie, we have a sort of informal examination/confession with the use of the basal thermometer to determine the best time for them to conceive a child.  Again ironically, this is the conception of Cal (so while science was carefully used, too much genetic recombination was already set in motion). 
This interplay of knowledge and pleasure, though it allows for an intensification of the pleasures associated with sex, also produces a certain truth about sex.   That brings me back to the initial ethical conundrum.  Because the sibings were certain that no harmful truth would ever come from their union, they were able to have sex, guilt-free, and I think our discomfort with the specific situation may be unfounded.  It was important though, that they be certain, as the physical result of a sexual union— a child— cannot be denied in reality.  It is what is literally produced, an actual piece of truth created from a sexual union.  It is science, indeed, genes combine and recombine, but truth be told, Desdemona’s spoon never did lie.