Here is a line of thinking which I have tried developing for a few years, and I would greatly appreciate any help advancing it (or challenging it, for that matter).
The summary would be something like this: The moral demands of Catholicism represent ideal human behavior, including thoughts and actions. Not so groundbreaking, though you may be someone reading this who would disagree.
It becomes interesting, I believe, when laid next to the modern approach to morality. Co-habitation is a fairly innocuous example, partly because many people who co-habitate do so out of a sense of necessity. It is more practical, they will argue. They often are not repulsed by the thought of marriage, but in no hurry either. Co-habitation is convenient.
Put another way: Few people approach the start of co-habitation with the same joyful expectation as most people enter marriages.
In that light, I have found few people who would disagree with the notion that living apart from one’s significant other (or fiance when things become more serious) should be preferred in a fundamental way to co-habitation. Now, some of these may also be Catholics, and here is the reason for raising this line of thinking at all: How does one persuade a modern Catholic adult to abandon his or her co-habitation plans (or lifestyle, if the plans are already in effect)? How does one persuade such a person concerning any moral issue, many which have become more powerfully influenced by efficacy (of the necessary and superfluous things, such as money and gratification) than by clear moral statutes?
Perhaps the appeal starts with a search for the pre-Fallen ideal, the very rational guidelines established by, if you’ll pardon the expression, the Manufacturer. If that search yields a common result, then a new appeal begins: An appeal to courage, to discipline, to excellence.