Historically the church decreed certain behaviors as sins in order to stop what it considered a detrement to society. I suspect a lot of marriages took place in those days because of a couples' underlying desire to have sex. (No dessert before dinner.)
Some people believe having sex or living with someone will reduce their odds of getting married. According to Psychology Today statisically over 50% of the marriages that take place in the U.S. are between couples who have lived together. Simply put a marriage takes place when and if (both people) in the relationship want to get married.
Most dating relationships (don't lead to marriage) whether the couple has sex/live together or not. If they did we'd all be married to our high school sweethearts! Awhile back I wrote a hub on this topic.