Sex/Living Together Before Marriage
A sexual relationship is so much more than just sex. When you become intimate with someone, you are bonding yourself emotionally, spiritually and physically to a person in a way that is only designed for marriage. A great deal of hurt can occur when you physically give yourself to someone that is not your spouse. God is lovingly warning us against this hurt as he tells us to, "Let marriage be held in honor among all, and let the marriage bed be kept pure, for God will judge the adulterer and all the sexually immoral" Hebrews 13:4.
Hebrews 13:4 talks about how God designed sex to be something beautiful in marriage, and desires to protect us from the hurt that occurs when intimacy precedes marriage vows. This also goes for living together; it is unwise to put yourself in a situation that will lead to obvious temptation.