I fail to see the logic that justifies incest as truly erroneous. Yes, it can result in deformities and defects. However, since when has incest implied that there will be a pregnancy? It simply doesn't imply that; it only implies there will be sexual relations between the two, nothing more, nothing less. I see no logical error in two adult relatives having consensual intercourse with contraception.
Homosexuality, incest, and pedophilia are society's sexual taboos and wrongdoings. In essentially every society these are ingrained taboos and wrongdoings. We are slowly, although, gradually realizing there is no valid logic to dictate them as truly erroneous. These debates are to be addressed with logic, not customary principles and unsubstantiated opinions.