The difference lies in the way Jesus himself carries these realities in his body and in the body of his church. With respect to the animals, we saw via Peter’s address that they are gathered up by Christ as reconciled creative distinctions that he bears in his own death. But with respect to the laws of sexual relation and family, we see the legal particulars, much as in the Sermon on the Mount, taken up by Christ and passed on to his church in an almost exaggerated fashion, renewed and refocused. We see both these things, however, not according to a logic of categorisation, but according to the discernment of time as the Scriptures have molded them in God’s good will. (Radner, Leviticus, 200)
Admittedly that last sentence is a little cloudy—which is true for every sentence in Radner (and, as it happens, Robert Jenson) in which the word “time” appears—but the point he is making is both right and important. Simplified to a criminal degree: why do we adhere to Leviticus on sex but not on shellfish? Jesus.