Incest
In history (as far as we know it) there were societies which allowed incestual marriages and individual acts of incest. Famously, the ancient Egyptian royal family practised incest, though not exclusively. It is thought that the frequency of early non-trauma deaths (such as Tutankhamun) was due to the effect of incest. We do know that incest was practised in their royal family. Cleopatra was married to her brothers, Ptolemy XIII and Ptolemy XIV. Roman records show that marriages between close siblings were common in Ancient Egypt.