There is no grey area. It's wrong, and if you disagree, your "morality" is below the level of human dignity.
Exactly. There are plenty of gay friends of mine who called me a bigot because when I hear about a mother and son in a sexual relationship, I want to throw up.
Wanting to throw up is one thing. Wanting to keep the activity illegal is another, and nobody has yet put forward a convincing argument in favor of that that doesn't just rely on the first point.
Birth defects as a result of incest isn't a good enough reason? These people do belong in jail if that's what it's going to take to keep them from reproducing. Heck, are we going to legalize incestuous marriage now if the parties are incapable of reproducing?
In cases like this, this is one pretty big example of where the "Disgusting" argument really does work in favor of banning certain kinds of relationships.