I turned to my trusted internet resource “Wikipedia” for the answer; it defined “living in sin” or “shacking up”, not surprisingly, as “living with and having sex with a person to whom one is not legally married.” But then Wiki further explained that these phrases are associated with negative attitudes of the church, but in modern times, are used in humor (meaning, derisively).
Yes, indeed, in our modern times, 'cohabitation' has been normalized. Consequently, many new believers will be in such relationships. How should we address them? I'm quite sure Jesus doesn't want us to throw stones at them. But to love and teach them. In the morally depraved city of Corinth, there were undoubtedly some 'cohabitating' converts. But with compassionate understanding of sexual temptation, Paul admonishes them to be married (1 Cor. 7:2). Maybe things haven't changed that much. But then, neither has the need to be compassionate, yet truthful (Eph 4:15). And that won't be easy in a culture that rejects all moral absolutes.