Laura Mitchell has a nice piece in which she describes the "transaction costs" of cohabitation. Couples that move in together make it all the more difficult to break up, because there is so much involved in ending the relationship. The relationships takes on a sense of inertia that speaks more to the costs of ending it than it does to the quality of the relationship. Those who choose cohabitation often drift into marriage because of this inertia and not because of sound choices. It's not surprising then that couples who cohabit before marriage are 1/3 more likely to divorce.