The structure of an average American's family tree is pretty simple. A mom and a dad, their parents and parents of their parents, their kids and then spouses of kids and their parents. Nobody was initially related to each other and unless it is a married couple who came from two different families, there are no sexual relationships whatsoever. In the Greek tragedy "Oedipus the King" , Sophocles wrote about the ruler of Thebes who crossed …showed first 75 words of 349 total…

…showed last 75 words of 349 total…he choose to live in torture by blinding himself. Even though this play was written a long time ago, the family structure is very similar to ours. It was a shame to marry a parent and it was even more dishonorable to have kids with that person. King Oedipus showed that it's impossible to have sexual relations within one family and that only death, shame and suffering could come out of a relationships like that.