If I want to, I can list dozens of reasons why consensual sexual relations between consenting heterosexual adults are almost always going to result in disastrous outcomes. The fact is, we can play this game with any kind of relationship. None of the "reasons" cited in this article are germane to the central question. In reality, provided the couple involved do not procreate, there seems nothing inherently wrong with incest provided it is indeed consensual and non-abusive. But let's be honest: how often can we truly say that about any relationship that persists over time? People withhold (a form of abuse), sometimes consent is questionable (she was drunk after an argument). There is thus no a priori reason why non-procreative incest should be any more, nor any less, problematic than any other relationship type.