Inbreeding is a universal taboo. Filial idiocy is its consequences in a huge percentage of the cases. That's where the taboo begins, but even protected sex with a relative -in many cultures even with a distant cousin- is considered strenuously abhorrent. The point of every living cell is to be looking outwardly (be extroverted) for a mate then to be looking within itself (be involuted) so that a broadening of its culture and of its genes takes place. Freud's view, if I remember correctly was something in the vicinity of incest equals autoeroticism; that is, one is, in effect, having sex with oneself, diminishing the worth of the other, eradicating what ego and superego the other has, turning that person into an apathetic (I think is the term he used) being, one that cannot feel other people's emotions. The wish of someone (father or mother) to have sex with their child, shows a rejection of the rest of society and a view that their "own being" is the only being good enough to have sex with. It is a narcissistic view, taken to its extremes. If there is a birth as a consequence, then that new person will also have a great many profound psychological problems, again to do with his/her relationship with the rest of society, the sort which I could never in my wildest dreams imagine.
Incest should be repulsed by all societies and sternly and unequivocally prohibited.
As for Peter Singer's view that having sex with animals is OK, it is something which he has since qualified with language such as, "so long as the animal is not hurt," or "the ethics involved are difficult to evaluate in such cases" and by saying this, I'm not advocating that sex with animals is ethically or morally good." I heard a radio interview with him, when he was here in Oz, not very long after he had made that lecture and, to me it sounded like philosophical babble -just a means of getting a headline or, better still, a book out. Very poor and disappointing stuff.
