Statistically, those who move in together before getting married are less likely to be successful and satisfied with their marriage, and are more likely to take part in extramarital affairs. An estimated 42% of couples who got married before moving in together report being happy, versus 28% who moved in before getting married.
More research seems to suggest that living together is the culprit; when you’re living together, it’s harder to leave that relationship because you share possessions in the house, a dog, and the ownership of the property. This seems to be counteracted by ensuring you are moving in together for relationship reasons (i.e. marriage or a life partnership of some sort), not for logistical reasons (i.e. the rental contract is done at the end of next month).