From a piece titled “Study Finds Cohabiting Doesn’t Make a Union Last“:

Couples who live together before they get married are less likely to stay married, a new study has found. But their chances improve if they were already engaged when they began living together.
The likelihood that a marriage would last for a decade or more decreased by six percentage points if the couple had cohabited first, the study found.
The study of men and women ages 15 to 44 was done by the National Center for Health Statistics using data from the National Survey of Family Growth conducted in 2002. The authors define cohabitation as people who live with a sexual partner of the opposite sex. . . .
The authors found that the proportion of women in their late 30s who had ever cohabited had doubled in 15 years, to 61 percent.

Mike McManus, who, along with his wife, has devoted much time to helping couples make marriage work, talked to NRO a bit ago about the harm and impracticality of cohabitation.