Those who moved in with a mate before engagement or marriage reported significantly lower quality marriages and a greater potential for split-ups than other couples. For instance, about 19 percent of those who cohabited before getting engaged had ever suggested divorce compared with just 12 percent of those who only moved in together after getting engaged and 10 percent of participants who did not cohabit prior to the wedding bells.
If you want to read a great article on the Church's perspective, Fr. David (pastor of St. Mary's Catholic Center), has one you should read (note: it is a .doc format).
In a similar study, researchers (they should have just asked someone who was married) found that love isn't the only thing needed to make a marriage work.
A couple's age, previous relationships and even whether they smoke or not are factors that influence whether their marriage is going to last, according to a study by researchers from the Australian National University.