From "The Negative Effects of Cohabitation," an article by Linda J. Waite in the winter edition of the Responsive Community:

A substantial proportion of cohabiting couples have definite plans to marry, and these couples tend to behave like already-married couples. Others have no plans to marry and these tentative and uncommitted relationships are bound together by the "cohabitation deal" rather than the "marriage bargain." In fact, couples may choose cohabitation precisely because it carries no formal constraints or responsibilities.

But the deal has costs. The tentative, impertinent, and socially unsupported nature of cohabitation impedes the ability of this type of partnership to deliver many of the benefits of marriage, as does the relatively separate lives typically pursued by cohabitatng partners. The uncertainty . . . makes . . . specialization with this partner much riskier than in marriage. Couples who expect to stay together . . . can develop some skills and let others atrophy because they can count on their spouse . . . to fill in where they are weak. This specialization means that couples working together in a long-term partnership will produce more than the same people would working alone. But cohabitation reduces the benefits and increases the costs of specializing--it is much safer to just do everything for yourself since you don't know whether the partner you are living with now will be around next year.