I must liken cousin marriages to interracial marriages here. These types of marriage restrictions were born out of the Eugenics Movement
. It's the movement that brought us segregation sterilization, strict immigration restrictions (i.e. cousin marriage restrictions), and finally the Holocaust. These laws were based upon "solid" science at the time and supported by top scientists..
Interracial marriages are still considered odd, and is another cultural taboo (in the US anyway) . The good news is that laws rooted in bigotry and ignorance can and must be changed! Perhaps laws banning cousin marriages are one of the laws that must be challenged as in the Loving v. Virginia case.
Love is an intoxicating gift. Love will cause people to do things they might not do normally. For instance, cousin & interracial marriages. Love sees through cultural boundaries. Love doesn't care what someone else may think. The "normal" boundaries seem insignificant when you are in love.
Everyone reading this has first-cousins in their family tree. If there were any truth to the myth of cousins and birth defects, we would all be in wheelchairs -- if we could figure out how to make them.