How do you define “cohabitation”? Married couples cohabitate. So marriage is not sinful because it is a bonding of two persons who are committed in a loving relationship as opposed to a relationship of mutual exploitation.
If by “cohabitation” you mean people who live together for the sake of sharing expenses, that too is not immoral or sinful.
If by “cohabitation” you mean people who live together and fornicate, and are not committed to a lasting love but merely use each other until they are bored with each other and split, that is by any definition a low-life relationship, the products of which may or may not (but often are) sexual diseases, abandoned children, and corroding character formation. Yes, that is immoral, but more to the point sinful because it is a stain upon the building of a healthy personal life and a productive society, and clearly is an assault upon Christian values. That is the Christian litmus test.
What is your litmus test that this third type of cohabitation is not immoral?
The fact that you are not a Christian? That’s not a litmus test.
You may have noticed, if you are old enough to notice the difference between the world of 1960 and 2016, that there are among blacks, for example, a horrendous decline in the quality of life. This is due largely to black men who cohabit with women and produce children who are fatherless and without a masculine model to follow in life. Such children, especially the male offspring, tend to be less discipline and motivated to becoming full fledged members of society, and often end up in gang warfare, as the statistics for Chicago and other great inner cities demonstrate.
Do you have a litmus test for arguing that this is a good or even acceptable fate for the blacks in America?