We all crave acceptance. We long for love. While we enjoy moments of solitude, we seek the company of others who will accept us for who we are. As humans we have an innate need for relationship.
As I've argued previously, this desire for relationship, this emptiness that we feel when alone, can be explained by resorting to the doctrine of the imago Dei (Genesis 1:26-28). Just as God in the Trinity is love and expresses love, humans created in His image must love and be loved.
Taken at face value the image of God in man explains why we surround ourselves with those we like and those who like us. It explains the urge to shower our affections on our pets. It's part of who we are. It also explains, however, why my German Shepherd Luther can never replace the relationship I enjoy with my wife. The image of God in me draws me to the image of God in her.
To better understand this phenomenon, we can turn again to the book of Genesis to see how this relates to the biblical construct for marriage.
After the creation of the apex creature, humanity, we find Adam at work in the Garden of Eden. He busily exercises dominion over the creation by sovereignly choosing the names of all the animals. As he surveys his domain, however, Adam longs for something more. He is not satisfied with the relationship he has with the animals. He is alone. God says that's not good (Genesis 2:18).
To rectify the situation, God determines to make a "helper suitable for him." God could have created a German Shepherd but He didn't. He could have created a golfing buddy but He didn't. No, the helper that Adam needed was a complementary being, one that fit the hole in his heart, one that provided what he lacked. God gave Adam Eve.
Here, too, we see God's plan for human relationships. Here we see God giving Adam a help-mate, a partner suitable to Adam's role in life. He gave him a woman. He gave him the real missing link, not a heretofore undiscovered biological primate, but the perfect human being to enable Adam to find fulfillment in relationship and to fulfill his place in the created order - be fruitful and multiply. Without Eve Adam was incomplete and incompetent. That, says God, was not good.
The account of the creation of Eve in Genesis 2 becomes paradigmatic, then, for our understanding of God's plan for our sexual beings. God provided the suitable helper, a woman, not women and not man. While men need male companionship in various seasons of their lives, while women long for an intimate friend to share their experience, only the male/female relationship satisfies our deepest longings. That's why God gave Adam the perfect mate, his perfect match - Eve.
Genesis 2 goes one step further. In Moses' account of the gift of the perfect mate, lasting union necessarily follows the initial encounter. The relationship between Adam and Eve does not stall at the level of simple co-workers. They didn't engage in casual sex to scratch some carnal itch. They united in primeval marriage. As it was intended to be Adam became joined to his Eve and the two became one - one flesh, one mind, one purpose (Genesis 2:24-25). As the passage explains, this was how things "ought" to be.
Jesus interpreted this passage in the same way. In Matthew 19 he speaks of the male/female bond. He reminds his apostles of the union of one flesh. And, then, Jesus explained the profound implication of this oneness. The relationship is not only physical, it is spiritual. "What therefore God has joined together, let no man separate" (Matthew 19:4-6).
Biblically-speaking, marriage is not a social construct. Marriage is not a constitutional right. Marriage is the first human institution ordained by God, designed by God, and fulfilled by God. We can choose to marry or remain single but we cannot choose to redefine marriage as we see fit. As God has revealed, marriage is: one man, one woman, one life.