Pondering this question lead me to the words of Elder David A. Bednar in his article entitled "The Doctrinal Ideal of Marriage."
In “The Family: A Proclamation to the World,” the First Presidency and Council of the Twelve Apostles proclaim “that marriage between a man and a woman is ordained of God and that the family is central to the Creator’s plan for the eternal destiny of His children.” This keynote sentence of the proclamation teaches us much about the doctrinal significance of marriage and emphasizes the primacy of marriage and family in the Father’s plan. Righteous marriage is a commandment and an essential step in the process of creating a loving family relationship that can be perpetuated beyond the grave.-- Elder David A. Bednar, "The Doctrinal Ideal of Marriage," Ensign, June 2006, 82-87 (bolded emphasis added).
Two compelling doctrinal reasons help us to understand why eternal marriage is essential to the Father’s plan.
Reason 1: The natures of male and female spirits complete and perfect each other, and therefore men and women are intended to progress together toward exaltation.
The eternal nature and importance of marriage can be fully understood only within the overarching context of the Father’s plan for His children. “All human beings—male and female—are created in the image of God. Each is a beloved spirit son or daughter of heavenly parents, and … has a divine nature and destiny.” The great plan of happiness enables the spirit sons and daughters of Heavenly Father to obtain physical bodies, to gain earthly experience, and to progress toward perfection.
“Gender is an essential characteristic of individual premortal, mortal, and eternal identity and purpose” and in large measure defines who we are, why we are here upon the earth, and what we are to do and become. For divine purposes, male and female spirits are different, distinctive, and complementary.
After the earth was created, Adam was placed in the Garden of Eden. Importantly, however, God said it was “not good that the man should be alone” (Gen. 2:18; Moses 3:18), and Eve became Adam’s companion and helpmeet. The unique combination of spiritual, physical, mental, and emotional capacities of both males and females were needed to implement the plan of happiness. Alone, neither the man nor the woman could fulfill the purposes of his or her creation.
By divine design, men and women are intended to progress together toward perfection and a fulness of glory. Because of their distinctive temperaments and capacities, males and females each bring to a marriage relationship unique perspectives and experiences. The man and the woman contribute differently but equally to a oneness and a unity that can be achieved in no other way. The man completes and perfects the woman and the woman completes and perfects the man as they learn from and mutually strengthen and bless each other. “Neither is the man without the woman, neither the woman without the man, in the Lord” (1 Cor. 11:11; italics added).
Reason 2: By divine design, both a man and a woman are needed to bring children into mortality and to provide the best setting for the rearing and nurturing of children.
The commandment given anciently to Adam and Eve to multiply and replenish the earth remains in force today. “God has commanded that the sacred powers of procreation are to be employed only between man and woman, lawfully wedded as husband and wife. … The means by which mortal life is created [are] divinely appointed.” Thus, marriage between a man and a woman is the authorized channel through which premortal spirits enter mortality. Complete sexual abstinence before marriage and total fidelity within marriage protect the sanctity of this sacred channel.
A home with a loving and loyal husband and wife is the supreme setting in which children can be reared in love and righteousness and in which the spiritual and physical needs of children can be met. Just as the unique characteristics of both males and females contribute to the completeness of a marriage relationship, so those same characteristics are vital to the rearing, nurturing, and teaching of children. “Children are entitled to birth within the bonds of matrimony, and to be reared by a father and a mother who honor marital vows with complete fidelity.