Bumping this to get some commentary going, but I have to say the author could use a logic course. It may well be true that it is always immoral to separate the unitive and procreative elements of sex. Indeed, volumes have been written on it. It may well be true that birthrates have fallen precipitously among non-muslims (and apparently is true in Europe, though not necessarily elsewhere). And the separation of the procreative and unitive aspects of sex may well be a factor in some regions - probably is, in fact.
Nevertheless, effect B (lower birthrate in Europe) is not "empirical" proof of premise A (the immorality of, for example, contraception), because if every couple banged out three kids THEN practiced contraception, the argument falls apart. Yes, it may still be immoral (count me in among post-1930 Protestants in this regard, though), but there are much better arguments for this premise than the "empirical" evidence cited. The author would be better off simply arguing that "we need more kids" based upon the data provided.
That might be considered a safe statement. Evidence that one could take as supporting the thesis that the teaching is "a teaching which is based on the natural law as illuminated and enriched by divine Revelation." Not conclusive in and of itself but evidence to be considered.
According to the Oxford English Dictionary (2nd Edition, 1989), empiric is derived from the ancient Greek for experience, which is ultimately derived from 'in' + ' trial', experiment. Therefore, empirical data is information that is derived from the trials and errors of experience. In this way, the empirical method is similar to the experimental method. However, an essential difference is that in an experiment the different "trials" are strictly manipulated so that an inference can be made as to causation of the observed change that results. This contrasts with the empirical method of aggregating naturally occurring data. (From Wikipedia, the Greek letters got garbled in the cut and paste so I edited them out.)
Given the tenor of the times in 1968 and the clamor about the population explosion, it seems doubtful any but the most far-sighted would have predicted a population implosion resulting from the wide-spread breakdown in the understanding of the nature of marriage. Although a consideration of the birth patterns and marital practices of the Romans during the twilight of the Empire in the West might have been a clue.
It's an interesting document: