I probably should start out by saying that I've got a proper, serious "ick" reaction to this as well, but...
In fairness, I, too, see where they are coming from. The argument for any relationship is that it needs two consenting adults, and in my view incest laws need to be subjected to closer scrutiny in this regard.
The problems with incest in my view are two-fold: a) as has already been mentioned, there is an increased likelihood of birth defects - however, looking at the actual science, it's not that much increased, plus not all incestuous couples would choose to have children.
b) and this seems to me a bigger problem, is that in any parent-offspring relationship, even with an adult son/daughter, there is always a chance that the powers are unevenly distributed. If a father were to marry his daughter there may be a possibility of coercion on the father's part. But again, it's difficult to make this a blanket reason not to allow such marriages, as this could hold true for any other relationship as well.
It is a tough one, that. So I don't think it deserves as much ridicule as it gets.