Argument: Inbreeding is increasingly risky and unsustainable over time
"Incest". FAQS.com - Besides reinforcing the incest prohibition, this rule prevents families from becoming culturally ingrown through continuous endogamy (marriage within a kinship group). Highly inbred populations havediminished reproductive capacity and have higher risks for hereditary disorders.
Allan Koay. "Incest: Age-old taboo." Star Online. May 5, 2008 - Members of a family have similarities genetically, which include recessive genes that cause defects and diseases. The risk of birth defects is therefore higher in incestuous relationships. And these recessive genes will have more opportunities to double and become a danger if inbreeding were allowed to continue for a few generations.