I remember talking about this in one of my previous soci courses. Basically, people who live together before marriage have a different mentality from people who don't. Mainly, those who cohabitate before marriage are generally more liberal in attitudes as opposed to more traditional 'waiting till marriage' people. These days people are waiting longer to get married. Instead of getting married right away, couples often cohabitate or live as 'common law.'
Quote from the article
U.VA Economists Challenge Notion that Living together befoer marriage leads to divorce
"Others use a cohabitation period to learn more about their partners," Stern says. "If, once they've learned more, they find they have a good match, they marry; otherwise, they separate without incurring large divorce costs. On average, their matches are not as good and they have higher divorce rates."
From Cnn story Study:Living together may lead to breakups
"It may not be the experience of cohabiting but the people who cohabit," said William Mosher.
"What we're saying about that is that we think that couples who cohabit before marriage may have different values than couples who do not," he said.
Couples who live together before marriage may be the type of people who are "more likely to consider divorce," he explained.