Argument: Hereditary risk of incest are same as some other couples
"The case against love: A recent legislation on incest". University of Oxford, Practical Ethics. April 7, 2008 - The strongest, though most controversial point, is the last one: the protection from hereditary diseases. In the case under debate the pair already has four children. Three of them are in foster care, two are handicapped. Leaving the serious problems raised by classifying someone as handicapped aside, it is to be noted that if this was the only point justifying the illegalness of incest, a consistent law would require sanctioning all sexual acts in which the probability to father handicapped children is raised as compared to some standard. This standard might be the likelihood to have handicapped children within averaged over the whole society or the likelihood for a single couple that, according to current standards, counts as perfectly healthy.
The current law is inconsistent in this respect as it allows partners of all ages to beget children – although there is evidence that the likelihood for genetic defects becomes enhanced with older parents. Also people with diseases known to statistically enhance the chance of having handicapped children are not curtailed in their mating behaviour.
Adam Graham. "Are you ready for incest?". Renew America. October 28, 2004 - A major argument against incest is genetics. If you allow two closely related people to marry, their offspring may be prone to some serious defects because he or she would have genetic defects and poor general health. However, this argument fails under the regime of gay marriage because many genetically flawed people who will have children that will develop deadly diseases are allowed to marry. In addition, with gay marriage, we say that reproductive potential between the marriage partners is irrelevant, so who are we to say that the potential of reproductive defects should stop people from getting married?
"Sexual Ethics: Consensual Incest". A Nadder. October 13th, 2008 - It causes birth defects -- finally a somewhat true statement (although apparently the best strategy is to actually marry your 3rd cousin). But if that's reason to outlaw incest we need to outlaw drinking/smoking during pregnancy (which we don't and it would be a big deal to do this). We'd also need to prevent 2 carriers of a genetic diseas from procreating. Finally despite what you hear on the news, we're not living in the 12th century. Sex ≠ babies. 2 sisters having sex, or a heterosexual incestuous couple using contraception will demolish the argument.
Joanna Grossman. "Should the law be kinder to 'kissin' cousins?". Find Law. April 8, 2002 - The prohibition of cousin marriages suffers from problems of both under- and over-inclusiveness--flaws that are usually fatal to a statute under heightened scrutiny. These bans are underinclusive in that they do not prohibit marriage in other cases where the risk of producing children with birth defects is significant. Carriers of diseases like cystic fibrosis, for example, are permitted to marry and reproduce with other carriers, even though resulting children have a 1 in 4 chance of developing the disease. For most individuals, the decision whether to marry and reproduce in the face of known risks to resulting children is left to their discretion.
"Inbred obscurity: improving incest laws in the shadow of the "sexual family". Harvard Law Review. June 2006 - "The rationale based on genetics is also suspect under current law: even setting aside nonprocreative intimate relationships, in no other legal realm does the government criminally prohibit two people from having children because their offspring are more likely to inherit genetic defects.30 Even though the risks of birth defects among the most closely related family members are significant,31 eugenics on the basis of physical or mental deformity has long been repudiated."
"Incest and the Law". Infernal Ramblings. Mar 18, 2007 - "The trouble with incest, I think, is that if you permit incestuous relationships, you are basically permitting people to reproduce despite incredibly high odds that their children will be physically harmed. (Genetics are a nasty thing for siblings in love.) [...] On the other hand, one could argue that we allow people with genetically-transmitted diseases to reproduce. Heck, it's not unheard of for deaf couples to specifically attempt to have deaf children just to satisfy themselves, so what's wrong with permitting an incestuous family?"