Marriage between Man and Woman is Essential to His Eternal Plan

Quotes:

It was Joseph Smith who taught me how to prize the endearing relationships of father and mother, husband and wife; of brother and sister, son and daughter. It was from him that I learned that the wife of my bosom might be secured to me for time and all eternity; and that the refined sympathies and affections which endeared us to each other emanated from the fountain of divine eternal love. … I had loved before, but I knew not why. But now I loved—with a pureness—an intensity of elevated, exalted feeling, which would lift my soul from the transitory things of this grovelling sphere and expand it as the ocean. … In short, I could now love with the spirit and with the understanding also.

Yet, at that time, my dearly beloved brother, Joseph Smith, had … merely lifted a corner of the veil and given me a single glance into eternity. -Autobiography of Parley P. Pratt, ed. Parley P. Pratt Jr. (1938), 297–98.
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. -David A. Bednar, Marriage is Essential to His Eternal Plan, June 2006

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. -David A. Bednar, Marriage is Essential to His Eternal Plan, June 2006

Eternal marriage is not merely a temporary legal contract that can be terminated at any time for almost any reason. Rather, it is a sacred covenant with God that can be binding in time and throughout all eternity. Faithfulness and fidelity in marriage must not simply be attractive words spoken in sermons; rather, they should be principles evident in our own covenant marriage relationships. As we consider the importance of our personal example, do you and I discern areas where we need to improve? Is the Holy Ghost inspiring our minds and softening our hearts and encouraging us to do and to become better? -David A. Bednar, Marriage is Essential to His Eternal Plan, June 2006

Marriage allows these different characteristics to come together in oneness—in unity—to bless a husband and wife, their children and grandchildren. For the greatest happiness and productivity in life, both husband and wife are needed. Their efforts interlock and are complementary. Each has individual traits that best fit the role the Lord has defined for happiness as a man or woman. When used as the Lord intends, those capacities allow a married couple to think, act, and rejoice as one—to face challenges together and overcome them as one, to grow in love and understanding, and through temple ordinances to be bound together as one whole, eternally. That is the plan. -Richard G. Scott, The Joy of Living the Great Play of Happiness, October 1996

 
Dallin H. Oaks, Marriage and Divorce

Jeffrey R. Holland, How Do I Love Thee?

Scriptures:

And the Lord God said, It is not good that the man should be alone; I will make him an help meet for him. -Genesis 2:18, Holy Bible

Nevertheless neither is the man without the woman, neither the woman without the man, in the Lord. -1 Corinthians 11:11, Holy Bible

Definitions:

Marriage - the state of being united to a person of the opposite sex as husband or wife in a consensual and contractual relationship recognized by law

Essential - of the utmost importance: basic, indispensable, necessary.
-Merriam-Webster.com