Whenever two people hook up in pop culture, they have sex. Without showing some of the unpleasant realities of premarital and extramarital sex, it is drawn up to be a wonderful, fun recreational activity. Outside of marriage, sex can have harsh consequences. Pregnancy, sexually transmitted diseases, guilt, insecurity, and shame can follow. A relationship based on lust can only last as long as the two are physically close and find each other sexually attractive.
Love is a choice.
It's a commitment. Although feelings will accompany love, and although sex will be a part of marriage, a lasting, healthy relationship cannot be based on these things. The Bible says that God is love. God, as our designer and creator, made us with needs for love. It's because God designed us for an unconditional love, and we, as people, are flawed.