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.
The two relationships are just too different. In marriage, children become an important factor, with the committed desire to stay together in order to raise the children. Clearly, people who are insecure about relationships or who have a fear that they may be left or rejected by someone or can't commit themselves fully, often have cohabitation after cohabitation after cohabitation. Those people should quite frankly continue to cohabit because it probably works very well for them. They should never get married because it's obvious that they don't know how to make a commitment.