In book six of his De Genesi ad Litteram (Literal Commentary on Genesis) St. Augustine addresses the question of whether human souls might have been created simultaneously with the rest of the world at the beginning of creation, with their bodies being formed only later on. Augustine gives two arguments against this view:
first, because of that completion of God’s works, I do not see how these could be understood to be complete if anything was not there established in its causes which would later on be realized visibly; secondly, because the difference of sex between male and female can only be verified in bodies. (De Genesis 6.7.12)
According to Augustine’s second argument, sexual differences are not psyche-logical differences, but physio-logical differences.
Compare this now with Tolkien’s account of the Valar in the Ainulindale:
Therefore the Valar may walk, if they will, unclad, and then even the Eldar cannot clearly perceive them, though they be present. But when they desire to clothe themselves the Valar take upon them forms some as of male and some as of female; for that difference of temper they had even from their beginning, and it is but bodied forth in the choice of each, not made by the choice, even as with us male and female may be shown by the raiment but is not made thereby.
For the Valar, “sexual” differences are more than–because prior–to bodily differences, being a mater of “difference of temper” that is then “bodied forth” afterward in the physical appearance the individual Valar choose for themselves.