In some communities many couples live together without getting married. When a person becomes a Christian he will want to legalize his marriage. In this way, he has a better testimony in the community, he gives protection of the laws of the land to his family, and he honors Christ.
God's Word is very strong against any kind of sexual relation outside of marriage. He strictly prohibits any flirtation or illicit love affair by either marriage partner with another person. One of the great blessings of a Christian home is the fact that husband and wife can trust each other when they know that each is faithful to the other in thought, word, and deed. As they serve the Lord together, it becomes a "little bit of heaven," free from the deception, suspicion, jealousy, and unfaithfulness that wreck so many homes.