It's against the Law to kill---but the Church made exceptions to that.  It is against the Law to steal---but there are exceptions to that as well.  The Law is to be there to serve the people---not the people to be made servants of a concept that fit another time, another place.  Vatican II moved from the old classical concepts into historical consciousness.  We need to do the same in dealing with peoples who live in diverse circumstances.  Specific moral norms by their very nature admit of exceptions because of the influence of individual circumstances.
---------Citations
Bernad Lonergan, COLLECTION [New York: Herder and Herder, 1967] pp. 252-67 and "A Transition from a Classicist Worldview to Historical Mindedness," in THE ROLE OF LAW IN THE CHURCH TODAY, ed. James E. Blechter [Baltimore: Helicon, 1967] pp. 126-33.