I like how most people in this tread claim that the children will be born with some sort of deformities, yet provide not a single shred of evidence beyond hearsay. I'm not necessarily for incest, but I also heard that incest greatly raising birth defects is a myth. After a browsing the internet for awhile, i came up with a couple different studies, surrounding Pakistani couples that actual allow first cousin marriages, and it says that the increase in infant death rate is 1.4% and the increase in birth defect is 2%. To that i'd say, so what. That is not a significant amount to warrant such a strong reaction to. Let me explain.
You have to follow me on this one a little, but hear me out. As females get older, the risk of having a child with Down's Syndrome jumps. From age 20 to 31, the chance doubles. From 31 to 35, it doubles again and it doubles every three years after that so forth. So as you get older, as a female, you have a larger chance of having a child with down's syndrome. After age 45, the chance of having a child with down's syndrome roughly equals the chance of a child from a incest union having any birth defect. So if you ban incest based on messing up the offspring than logically you would be for putting a maximum age limit on when you are allowed to conceive period. Another idea would be the increase from drinking while pregnant, which i do not think is illegal.
My point is that the genetic justifications for these laws don't hold any water.