Minor point: I find Julie-and-Mark-like examples silly because they ask for a moral intuition about a case where the outcome is predefined. Our moral intuition makes arguments of the form "behavior X usually leads to a bad outcome, therefore X is wrong". So if the outcome is already specified, the intuition has nothing to say; nor would we expect it to, since the whole point of morality is to help you make decisions between live possibilities, so why should it have anything to say about a situation that has already happened/cannot be altered?

Or to put it another way, I'm surprised no one said something to the effect of "Julie and Mark shouldn't have had sex because at the time they did they had no way of knowing that it would turn out well, and in fact every reason to believe it would turn out very badly, based on the experiences of other incestuous siblings."