There are certainly cases where there is no harm in such an arrangement. A live-in housekeeper or medical aide might share a home with a single parent or disabled person; a brother and sister might share living expenses; male and female students might share a dormitory.
The Church's prohibition is against premarital sex (whether the couple lives together or not) and also against scandal. A couple living together would have to consider how close quarters could be a temptation to sin, and how their arrangement would appear to others. Though perhaps unfair, an arrangement that causes others, especially children, to believe that immoral conduct is condoned should be avoided.