Argument: Marriage is a contract with little sanctity for incest to violate
"Inbred obscurity: improving incest laws in the shadow of the "sexual family". Harvard Law Review. June 2006 - "these incest laws treat as forever inviolable the two norms of marriage: it creates family and it is the only sexual relationship that may take place within that family. But the effects of marriage are not treated as forever inviolable in other areas of modern law. Indeed, the idea of marriage having everlasting effects recalls the highly 'status'-like position marriage embodied in past eras and contradicts the progressive treatment of marriage as more 'contractual.' Although marriage today is certainly not exclusively contractual,81 laws regulating marriage have moved over time in this direction,82 such as by allowing divorce and then no-fault divorce, and by allowing and enforcing premarital contracts."