Most children born to close relatives are healthy. There are attractive, outgoing, popular, successful, wealthy, educated people who have been, or are still involved with a sibling. Even so, sibling consanguinamory and reproduction are two different things. For most people, you don't have to go back very far in their ancestry to find it, whether the official family tree reveals it or not. It is true that in general, children born to consanguineous parents have an increased chance of these problems than those born to nonconsanguineous parents, but the odds are still minimal. Adults don't do this. Some siblings share this throughout their lives, some starting late in life. There's no good reason to have laws against consensual incest. Abuse and sex are two different things. Unless someone is willing to deny reproductive rights and medical privacy to others and force everyone to take genetic tests and bar carriers and the congenitally disabled and women over 35 from having children, then equal protection principles prevent this from being a justification to bar this freedom of association and freedom to marry. Also, marriage automatically provides for next-of-kin status, which is especially important when there is some discord between one or both siblings and other siblings or their parents or grown children. But where it is, the laws should be changed. To question if consent is truly possible in these cases is insulting and demeaning. It is the last one that people tend to be thinking of, usually, when they repeat this myth. For most, the involvement is for a season and it will pass. Birth defects can be the result of injury during pregnancy, substances ingested during pregnancy, environmental factors, or genetic problems. Some people say such laws are needed to prevent societal collapse due to everyone making mutant babies with their siblings.