People that are living together, in general, have higher divorce rates. First, people willing to live together are more unconventional than others and tend to be less committed to marriage as an institution. These two factors make it easier for them to leave a marriage later, if it becomes unsatisfying. Second, cohabiters value autonomy more and marriage involves less autonomy for both partners than living together. Finally, cohabiters are more likely to accept divorce and their enthusiasm for marriage and childbearing tends to dwindle the longer they live together.
Serial cohabitation is the biggest problem.
The experience of dissolving a cohabiting relationship generates a greater willingness to dissolve later relationships. These individuals may also have a relatively low tolerance for unhappiness in a relationship. On the other hand, cohabitation when both people plan to marry each other as a prelude to marriage during the engagement period is different from serial cohabitation or living together as an experiment to see if it will work. The problem with that is cohabitation is never going to be the same as marriage. Marriage is an institution that has roles as husband and wife.