God has called all humanity to “image” him by governing and filling the world in goodness—the creation mandate (Genesis 1:26-28). Eve was created to be Adam’s “helper” in this task, showing marriage to be the key means of its fulfillment (Genesis 2:18-23).
The three aims of marriage are therefore stewardship—utilizing one another’s gifts and the world’s resources for the common good, procreation—raising the next generation to do likewise (Malachi 2:15), and godliness—doing these things for Christ and through emulating him (Ephesians 4:22-24).
The primary goal of marriage is not therefore to offset loneliness, but to provide a family cell or team that will serve God. Yet companionship is key in ensuring the unity of relationship needed to properly fulfill this goal, and is deepened through the delight of working together in it and through the bonding intimacy of sex (Genesis 2:20b-24).
This working together manifests itself in different but complimentary roles: As head, the husband is to care for and nurture his wife, spiritually and physically, so that she grows to maturity in Christ and expresses her gifts fully in fulfilling, with him, the aims mentioned (Ephesians 5:25-33, cf. 4:12-13, 15-16). And she is to freely submit to his loving authority whenever he requires it, respecting his role and wisdom (Ephesians 5:22-23, 33). The husband is therefore particularly responsible before God not just for himself, but his wife too.
It is in these ways that God is "imaged" within marriage in a special way. He is imaged as Creator as the wife gives birth to new life. And he is imaged as Saviour by the husband leading and caring for his family.
Marriage may be defined as a covenant of companionship that is heterosexual—because sex and marriage were intended for procreation not same-sex friendship, and the roles required for marriage are complimentary not the same, sexual—in developing the one-flesh unity needed to fulfill all three aims, exclusive and lifelong—in ensuring this unity and providing a stable context for the aims to be met, and public—in establishing a new family unit within society (Genesis 2:24).
All this means that any potential marriage partner must not only be of the opposite sex, and someone we are attracted to, but also a committed Christian (Deuteronomy 7:1-4, Nehemiah 13:23-27, Ezra 9:13-15, 1 Corinthians 7:39). For without the Spirit, the non-Christian is neither willing or able to consistently fulfill the roles of marriage or achieve its aims (Romans 8:5-8).
Many mixed marriages endure more conflict, and see the faith of children seriously hindered by the mixed messages received. Indeed, the Christian partner is rarely able to flourish themselves, and often falls away (Jos 23:12-13, 1Kgs 11:1-6).
We must set ourselves apart from the naive mindset that assumes fulfillment cannot be found outside marriage. It is surely preferable to be even unhappily single than stuck in a permanent but unhappy marriage (Matthew 19:9-10).
Paul actually urges us to prioritize singleness because of the greater opportunity it gives for worry-free and undivided devotion to Jesus (1 Corinthians 7:25-35). For this reason he suggests that even Christian marriage should only be entered into when two friends find themselves sexually attracted to one-another, and would otherwise sin (1 Corinthians 7:36-38).
(Key passages: Genesis 2:4-25, 1 Corinthians 7)