This is from the "Catholic Update" web site;
Higher risk of divorce. Cohabitors who do marry 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 percent higher for cohabitors than for non-cohabitors. The divorce rate is even higher with previously married cohabitors and serial cohabitors (those who have had several cohabiting experiences). There are some indications that the divorce rate is higher for couples who live together for a longer period of time, especially over three years.
This is a pastoral letter from an American bishop: here (thanks to At Home in My Father's House, here)
This is from a blog dedicated especially to woman and beauty advice;
Scientists discovered that the couples who cohabit before marriage are more likely to get divorced later compared to those who start living together only after getting married.
Moreover, those who lived together before marriage and then tied the knot were less satisfied with their life in marriage, The Psychological Science reports. Psychologists also embarked on finding out why the pairs decide to cohabit and their poll revealed the main reason was the need to spend more time together.
The second reason was less financial pressure by supporting household together. By the way, these findings disagree with common belief that pairs start living together to test their relationship.
Cohabitation and irregular marital situations are together one of the main problems faced by Catholic parish priests today. We need clear leadership from our bishops. Without this, many Catholics are left confused because, unfortunately, they hear different things from different priests. For example, in the pastoral linked above, Bishop Sheehan states that those who are cohabiting or in irregular marriages may not be commissioned as extraordinary ministers. They are certainly not to be admitted to Holy Communion. There are cases that have been resolved in the "internal forum" (though this has often been abused), and some couples in situations that are certainly much less than perfect are allowed, with permission, to receive Holy Communion. Because such cases are kept confidential as regards the details etc, other parishioners can easily be confused and even bitter at what they see, but do not understand. The damage that has been caused by the refusal to marry or by people simply ignoring the Church's teaching in this area cannot be properly estimated on this side of the grave. For instance, how many sacrilegious communions are there? Such things seriously damage the spiritual health of the whole church and each parish community. The damage continues because no one has the guts to speak out about these things, and the lead MUST come from our bishops.