The practice of incest was widely prevalent in ancient societies mainly the royalty. The main purpose was perpetuation and preservation of the royal lineage. However today, incest is regarded as a cultural taboo. Selected societies regard it as a crime and the offence is punishable by law.
Incest is downright gross. But, this is not the sole reason for the prohibition. There are biological reasons behind the ban of incestuous relationships. Children born to parents with a very close genetic relation have an increased tendency of developing congenital disorders, defects and disabilities. It could also result in death.