Sex outside of marriage shows disrespect for the sacrament of marriage, the sacredness of sex, and human dignity. We have good reasons for living together before our wedding. The Church cares for you as a parent cares for a beloved son or daughter. Knowing that cohabitation increases a couples' chance of marital failure, the Church wants to protect you and preserve your happiness. Besides, most couples don't really evaluate the reasons they give to justify their decision.
Married life is sometimes inconvenient and even demanding.
Cohabitation for convenience is poor preparation for that kind of commitment. Research bears this out. One study, conducted by researchers at the University of Chicago and the University of Michigan, concluded that couples who cohabit tend to experience superficial communication and uncommitted decision-making once they are married. Sure, you might save the price of monthly rent, but you're sacrificing something more valuable. Engagement is more than just time to plan the party.