Why I believe incest is (usually) wrong, but natural.

Obviously I have no problem with incest, the only thing I have a problem with is the purity of the blood, especially when it comes to children. I believe that when parents sleep with their children, there is a higher chance of children with defects (if they decide to have children), especially if they are related through blood. I don't like this because the children may not be born healthy, and they don't have a choice, I would be much more tolerant of incest if it's between siblings or cousins, as long as they're not having children.