I know of a pastor who places in his bulletin a requirement for engaged couples going through his marriage prep program. He requires that if they are cohabiting, that they live separately until their wedding day. Why? In his words, "it is sinful, causes scandal, and leads to greater likelihood of divorce". Of course he's spot-on correct.
Here's another consideration. Since fornication is objectively a mortal sin, would that not mean that at least some of these cohabitating people have mortally sinned? Some, out of ignorance of Church teaching, may not be fully culpable, I realize. But for those in the state of mortal sin, it is my understanding that no grace from any other sacrament will be of benefit to them until their mortal sin is absolved in the Sacrament of Confession. Would that not include grace from the Sacrament of Matrimony?
The internet is now carrying the news that Pope Francis will marry twenty couples this coming Sunday including "those who are already cohabitating". That is troubling. It doesn't say "those who had been cohabitating". The Telegraph speaks in present tense, not in past tense.
I suppose by now these couples have already been selected. Have they been required to cease their mortal sin? Have they been required to confess their mortal sins of fornication and receive absolution from a priest? If not, they'll most likely not receive the sacramental grace that otherwise they might anticipate from Matrimony. Moreover, they'll most likely receive Holy Communion at these weddings while in mortal sin, right? That sounds like two sets of sacrilege right there!
As I read this article, I see no sign of these couples righting their relationships with God and the Church. I hope I'm wrong in this, but if not, we're looking at scandal of breath-taking proportions with the potential of poor souls careening to hell due to bad example and poor teaching.