Pardus said:
If noone gets pregnant and both are fully able to concent (age, etc), what is wrong with incest?
The main advantages of endogamy are strengthening family ties and conserving the family's wealth.
The main advantages of exogamy are genetic diversity and the creation of alliances with other families.
Whether inbreeding causes genetic problems depends on the genes involved. Cleopatra was considered beautiful and intelligent, but it's likely that her paternal grandparents were brother and sister, and her maternal grandparents were uncle and niece -- and the maternal grandfather was brother to both the paternal grandparents. Her great-grandfather (times three), Ptolemy VIII, married both his sister
and her daughter by another brother. Yet the dynasty lasted nearly three hundred years. On the other hand, there are genetic problems like hemophilia that can be carried by only one parent, and others like sickle cell anemia and cystic fibrosis that can be expressed even though the parents have no known relationship.
If we are to believe Genesis, Abraham married his half-sister Sarah, and his brother Nahor married his niece, Milcah. Abraham and Sarah's son Isaac married his first cousin once removed (times 2) and also first cousin twice removed (times 2), Rebekah. Isaac and Rebekah's son Jacob married two of his first cousins, who were sisters to each other. It gets complicated to try to calculate all the ways Reuben and Joseph were related to each other:
Bottom line: the problem with incest is cultural. It's not something that people one knows would do.