Why? Gender or biological sex are qualities that don't have spiritual significance. The angels don't have these qualities; though physical creatures do (not all, there are plenty of organisms which are agendered or entirely non-sexual). And as it pertains to human beings, the Apostle St. Paul has made it quite clear "there is neither male nor female" in Christ. In the beginning God made human beings in His image, both male and female; what is of spiritual significance isn't in the expressions of our chromosomes, or our biochemistry, or in our genetics at all. But in that we, being human, bear the Divine Image, created for communion with God, created to bear God's image and likeness to the rest of creation and reciprocally bear creation's praise to the Creator.
N.T. Write describes the Imago Dei as man created as an "angled mirror" reflecting God to the rest of creation, and reflecting the rest of creation back to God; bearing the image of God by caring for creation and bearing worship to God as a rational creature on behalf of creation. Here, the language of "kings and priests" which we see at many points throughout Scripture, is poignant. Human beings were made to "have dominion" but such is a godly dominion of love and service as revealed in Christ; not the reign of terror which we have seen on account of the Fall. The earth is God's, and we were created to care for it and in a priestly way offer up the "sacrifice of praise". Such things will be restored in the Age to Come (and even more greatly than what we behold in Genesis 1-2), for God will at long last make His full habitation with us and we with Him.
To be human is spiritually signficant. But to be male or female, biologically, is just how our human genome is expressed in the function of procreation. As far as the more complicated debates, and hotly controversial subject, of the meaning of gender in present times, that is a topic beyond scope here I think.
But even as God made us in His image, and though that image marred by sin and death and held under the bondage of sin, death, hell, and the devil, has been set free in Christ; so in Christ our maleness and femaleness are of non-importance as it pertains to these things. It is that we are human, and in Christ the image of God is restored to humanity, and the fullness of humanity in Christ is ours by the gift and work and power of God. For having been united to Him, having been made a new creation in Him, by having the promises of God on account of Him, the work began will continue, to conform us to the image of Christ, to be human like Christ, and ultimately share in what that means in the ultimate sense: On the day God makes all things new, the resurrection of the body and the restoration of all things.
-CryptoLutheran
