v0.2 of myself was Catholic; subsequent revisions have removed this feature from my code base. In any case, I went through Catholic marriage prep, and was married in the Catholic Church. At the preparation retreat, which is obligatory if one wants to be married in a Catholic Church, they stressed quite strongly that cohabiting (living together before marriage) drastically increased the likelihood of divorce. This would be followed by an appeal to separate, even for a short time, before marriage to "reset" the relationship on good grounds and do things the Catholic way.
The United States Conference of Catholic Biships (USCCB) has a report (An Information Report on New Realities and Pastoral Practices) featuring some claims about this:
- When cohabitors do marry, they are more at risk for subsequent divorce than those who did not cohabit before marriage. In the United States, the risk of divorce is 50% higher for cohabitors than non-cohabitors. In some western European countries, it is estimated to be 80% higher. (Bumpass & Sweet, 1995; Hall & Zhao, 1995; Bracher, Santow, Morgan & Trussell, 1993; DeMaris & Rao, 1992; Glenn, 1990) 
- When previously married cohabitors marry, their subsequent divorce rate is higher than that of cohabiting couples who have not been previously married. (Wineberg & McCarthy, 1998; Wu, 1995; Bumpass & Sweet, 1989) 
- There is some indication that the divorce rate is higher for people who cohabit for a longer period of time, especially over three years. The data on this are mixed. (Brown & Booth, 1996; Stets,1993; Thomson & Colella, 1991) 
- Cohabitors who marry break up in the earlier years of marriage. Cohabitors and noncohabitors have the same rate of marriage stability if the marriage remains intact over seven years. (Lillard Brien & Waite, 1995; Thomson & Colella, 1991; Bennett, Blanc & Bloom, 1988) 
- Cohabitors who do choose to marry appear to be of lesser risk for later divorce than those cohabitors who choose not to marry would be. They appear to be the best risk of a high risk group. (Bumpass, Sweet & Cherlin, 1991; Bennett, Blanc, & Bloom, 1988) 
So, fairly well cited claims, but I'm a bit skeptical. There's the obvious personal component, as I no longer espouse the views of Catholicism, but there's a purely skeptical component as well in that the Catholic Church stands to gain support for its teachings if it can show evidence to support their benefits... and lo and behold, they have found evidence to support their teachings.
But this strikes me as quasi-related to evidence that condom distribution, after all, is somehow increasing the spread of HIV/AIDS in Africa. In other words the pope was right.
Questions: So, I'm curious. Is cohabiting increasing the probability that those who move forward from it to marry will divorce? Is any sociological/psychological mechanism known for why this might be? Are there credible sources with views divergent from those above?
From an intuitive point of view, I'd wonder why cohabiting might give couples a period of time to work out potential conflicts that might arise from constant close contact and interaction rather than entering blindly and being surprised/overwhelmed.
Sources (first listed of each bullet):
-  Bumpass & Sweet (1995), The changing character of stepfamilies: implications of cohabitation and nonmarital childbearing. (LINK).
-  Wineberg & McCarthy (1998), Living arrangements after divorce: cohabitation versus remarriage. (LINK)
-  Brown & Booth (1996), Cohabitation Versus Marriage: A Comparison of Relationship Quality. (LINK)
-  Lillard Brien & Waite (1995), Premarital cohabitation and subsequent marital. (LINK)
-  Bumpass, Sweet & Cherlin (1991), The Role of Cohabitation in Declining Rates of Marriage. (LINK)