The Bible does not use the modern term cohabitation (living together before marriage), but it is explicitly clear on the moral standard of sexual purity and avoiding the appearance of evil (1 Thessalonians 5:22). For Christians, living together before marriage is discouraged because it creates a constant temptation for sexual sin (fornication) and lacks the covenant commitment and security that God designed for marriage. Cohabitation often treats marriage as a trial period rather than a sacred, lifelong covenant, which contradicts the biblical view of commitment.  

Yes, marriage is necessary. For Christians, living together (cohabitation) is not a substitute for the public, lifelong covenant God established (Genesis 2:24). Marriage is necessary because it honors God's design, provides covenantal protection, and is the only context where sexual intimacy is not considered sexual immorality (Hebrews 13:4). The unmarried Christian is commanded to practice purity and self-control (1 Thessalonians 4:3–4).