In the context you presented your arguement, it is a very valid arguement. Birth Defects also occur in non-incest cases.
So, shall YOU make all sex illegal?
This is why the bride and groom have blood tests before they get married, to check to see if their blood types will not conflict with each other if they chose to have children.
So, shall YOU make all sex illegal?
You are the ones who want to make sex between two consenting adults illegal, not me.