Gender is an Essential Characteristic of Individual Premortal, Mortal, & Eternal Identity & Purpose

 “[Gender] 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. … The unique combination of spiritual, physical, mental, and emotional capacities of both males and females were needed to implement the plan of happiness.” -David A. Bednar, Marriage Is Essential to His Eternal Plan, Liahona, June 2006, 51; Ensign, June 2006, 83

"Understanding these truths ought to make it easier for us to feel like a little child, not just as we read the proclamation but throughout our lives, because we are children—but in what a family and of what parents! We can picture ourselves as we were, for longer than we can imagine, sons and daughters associating in our heavenly home with Parents who knew and loved us. Also, we know that in the premortal world we were men or women with unique gifts because of our gender and that the opportunity to be married and to become one was necessary for us to have eternal happiness. But now that we are here we can picture ourselves home again with our Heavenly Parents in that wonderful place, not only as sons and daughters but as husbands and wives, fathers and mothers, grandfathers and grandmothers, grandsons and granddaughters, bound together forever in loving families." -Henry B. Eyring, The Family, Ensign, Feb. 1998

Scriptures:

So God created man in his own image, in the image of God created he him; male and female created he them. - Genesis 1:27

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


Definitions:

essential - of the utmost importance: basic, indispensable, necessary
characteristic - a distinguishing trait, quality, or property
identity - the distinguishing character or personality of an individual
purpose - something set up as an object or end to be attained : intention
-Meriam-Webster Dictionary

premortal - life before this life
mortal - this life
eternal - life after this life
-Michael's definitions