"We should instead increase our sense of the horror an ancient Hebrew would have had at the idea of eating shellfish."We naturally recoil in horror when we think of eating a dog or a cat. I gather that this has nothing to do with any real moral sensibility but is merely a cultural taboo as there is no explicit condemnation of eating dogs or cats or hamsters in Scripture. However, it seems we should at least be able to empathize with the Israelites' feelings of disgust when thinking of eating lobster or shrimp because there was (at least at the time) a real and binding moral prohibition against their consumption? I'm not sure I personally have the moral imagination for that. One of my rare indulgences is a steak filet smothered with a lobster bearnaise. Now, if I truly believed it was "immoral", I'd refrain from eating it. To expect me to also try to summon some feeling of disgust for it is quite another thing. How does one force such a sentiment? (I don't think this applies only to food. We can, out of a sense of morality, avoid certain sexual vices, for example, while not having any particular distaste for the act itself.)
I wouldn't say that it's a genuine moral prohibition on eating shellfish. It's a genuine moral obligation to keep to the covenant, and this is one of the conditions of the covenant. We should see it as a violation of the covenant God had set up with them. But we shouldn't see it as a genuine obligation apart from the covenant, and the only feeling of disgust that's well-grounded is the disgust at disobeying God and at failing to maintain the distinctives of the covenant community.The sexual analogy might be helpful here, actually, because the idea in my mind of having sex with someone who isn't my wife shouldn't disgust me intrinsically. What should disgust me is the idea of breaking my commitment to my wife and the idea of disobeying God, not the idea (when isolated from that context) of having sex with that particular person.One way of recognizing this is that there's at least the possibility of death severing any marriage bonds preventing such a sexual relationship, allowing for a new marriage bond sanctifying it. It would be wrong to dwell on such a desire or to long for such deaths and such, but it shows that the object of the desire isn't intrinsically immoral. Contingent circumstances make it wrong, and those even might change, even if it's unlikely and not something even to seek after.