I think we, as a society, have a very bad habit of defining normal things just by majority view. More people agree, then it is normal otherwise wrong or sin. But just because most people believe in a thing doesn't make it right, just as the belief of everyone, in medieval times, that Earth is flat was wrong.
I think if two people, adults, are doing something with mutual consent, nobody should interfere. Variation is the key of universe. We can't define a rigid normal, because there will always be exceptions. And who are we to judge who is wrong? Just because you two have different opinion, you decide yours is right and theirs is wrong? Remember, commonplace is not always right. Once Sati also used to be "normal therefore right".
Now, the children are more likely to get genetic abnormalities. So, it is unfair for a child to have problems because it was not his choice. And remember, normal marriage of unrelated people can also have genetic birth defect, so it is not a very good criteria to prove incest wrong. But, the people should not take any chance.
So, if an incestuous couple is mutually agreeing, and practices protection to avoid pregnancy, I am perfectly OK with that.
Now, bring the flame war!