The natural law cannot and ought not be, in every instance, enforced by human laws. That is to say, human laws are a moral guide, but are not sufficient to make men moral.
Indeed, the lawgiver who wants to make other men saints by criminalizing all sin is actually stepping outside his authority over the community and is overreaching, according to Aquinas' understanding of the natural law. There are for Aquinas many things that are sinful and destructive, but which still ought not to be prohibited by human legislators. Such things, though evil, ought to be tolerated within society. Criminalizing such things causes reprocussions even more disasterous than the evil itself.
This struck me deeply.