You probably heard of this story: http://www.dailymail.co.uk/news/article-2922059/New-Jersey-legislators-rush-ban-incest-anonymous-father-daughter-reveal-plans-marry-children.html Daughter met her father after being estranged from him for 12 years, they have sex, and get engaged. I guess it makes some sense – he was never a proper father to her. Meeting him was probably like meeting a random older dude.
And while I know lots of people find it gross, “ew” is not a legal argument. Preventing the possibility of babies with birth defects also doesn’t hold up as a good reason. Not all incest results in babies. Plus people with defects and defect genes are allowed to procreate. Women over forty are allowed to procreate. If we want to prevent babies with birth defects, we should be consistent about whom we persecute. What should be done to prevent defective babies from being born? Should one force the mother to abort? Maybe sterilize the women in the risk group (women fucking their dad/brother, women over 40, women with genes for Harlequin syndrome…). Or maybe let the baby be born, but jail the parents and give the baby away for adoption. It doesn’t matter if the parents were married since 20 and had several babies already – having kids over 40 increases the risk of Down’s syndrome.
I don’t know why relatives would want to marry though. Don’t they already have rights to each other’s stuff, and things like hospital visits?