Incest vs. polygamy

Today in Slate there's an argument for why society should discourage first-degree incest. The main thrust of the piece seems to be broadly utilitarian, in that incest is destructive to the family unit and society has a rational motive in discouraging the practice. The reason that the argument is even made is because of analogies that some social conservatives make between incest and gay marriage. I'm not too interested in the argument against first-degree incest, because I think this is a practice which is aberrant because there are biological dispositions most humans have which make it unthinkable.* Though the genetic reasons are broadly well known, Steven Pinker reports on the psychological mechanisms which enforce the taboos in The Blank Slate. Of course there are exceptions to the rule. The first-degree incest taboos can be violated in the case of royal families which wish to preserve and accentuate their divine genealogical ...