[AlphaGam1019]

At Discovery Health, they have an article on the top 10 myths of marriage.

5. Myth: Couples who live together before marriage, and are thus able to test how well suited they are for each other, have more satisfying and longer-lasting marriages than couples who do not.

Fact: Many studies have found that those who live together before marriage have less satisfying marriages and a considerably higher chance of eventually breaking up. One reason is that people who cohabit may be more skittish of commitment and more likely to call it quits when problems arise. But in addition, the very act of living together may lead to attitudes that make happy marriages more difficult. The findings of one recent study, for example, suggest "there may be less motivation for cohabiting partners to develop their conflict resolution and support skills." (One important exception: Cohabiting couples who are already planning to marry each other in the near future have just as good a chance at staying together as couples who don't live together before marriage).

ETA:

9. Myth: Cohabitation is just like marriage, but without "the piece of paper."

Fact: Cohabitation typically does not bring the benefits — in physical health, wealth and emotional wellbeing — that marriage does. In terms of these benefits, cohabitants in the United States more closely resemble singles than married couples. This is due, in part, to the fact that cohabitants tend not to be as committed as married couples, and they are more oriented toward their own personal autonomy and less to the well-being of their partner.