Argument: Many things are bad for the family, but not banned like incest
David Archard. Sexual consent. Limits of Consensuality" (Book). 1997 - The problem with regarding such alleged harms [to the family] as supplying a sufficient condemnation of incest are two fold. First, it is possible to conceive of practices or sets of circumstances which do the same sorts and amounts of damage to the family but which do not appear to be subject to the same degree of extreme moral criticism. Consider, for example, intra-familial violence, parental subscription to very rigid or fundamental religious views, extreme poverty, ready toleration of and participation in criminality, excessive parental coldness or emotional indifference, and so on. It may still be insisted that incest is just so much worse in its consequences. But that looks implausible. Or at least it is unlikely that incest can be so much worse in its effects as to explain the view of it as totally different in moral kind from these other cases.
"Inbred obscurity: improving incest laws in the shadow of the "sexual family". Harvard Law Review. June 2006 - OBFUSCATED INCEST LAWS The social and legal norms described in the previous section take center stage in the enforcement (or nonenforcement) of incest laws, often leading to decisions based on inconsistent or tautological references to “marriage” or “family.” These norms hinder the law’s ability to operate on the basis of a legally consistent and normatively preferable scheme premised on nonconsent. The haphazard application of the law not only prohibits some relationships between consenting adults, but also muddies any message the law might send about the impropriety of abusing intrafamilial dependency relationships.