Yeah,I consider it a moral/scientific wrong and would keep legality out of the arena on this one,if for kicks/pleasure whatever,but for reproduction it's squirmy IMO.I'm not judging what people do or who they do as long as it comes from love and desire. On both sides.
Yes, the science is very clear about how important genetic diversity is for the health of any offspring from that act. Scientific research indicates that genetic diversity is the primary reason for males. Sexual reproduction is present in nearly all life forms.
That said, should there be a law against fucking one's cousin if it's between adults and mutually consensual? Mind you, I have greater distaste at the idea than in just about any other situation I can imagine that could take place between two human adults who both said yes. But I don't think my belief system should preclude it.