To say that the sexes are complementary is true - but it does not mean what most people think it means. Complementarity is both true and also represents an extremely radical re-framing of ultimate reality.
Complementary means each sex is incomplete without the other, the two sexes fit together like differently shaped halves of a puzzle; and the proper and highest unit of humanity is a dyad.
Complementarity means that neither sex is dominant overall - and that in specific contexts sometimes one, sometimes the other sex will be subordinate.
(This is not a mathematical equality of complementarity - simply that there is at-least-one vital domain of life where domination and subordination relations are reversed.)
Thus, with complementarity, there are domains of life in which women are properly and ultimately dominant over men - this sets complementarity qualitatively apart from theologies of patriarchy, or ideologies of male dominance.
Theologically, complementarity is a natural consequence of Mormon metaphysics which regards men and women as radically incomplete when individual (although saved as individuals); and completed by the dyad of celestial marriage which is necessary for the highest levels of theosis/ sanctification/ spiritual progression.
Indeed, Mormon metaphysics envisages all men and women as having been originally (before we became Sons and Daughters of God, before we were born to earthly parents as incarnate mortals) eternally-existing essences or seeds of being ('intelligences') which were either male or female.
So, no matter how far we go back in time or project forward; men and women have been, are and will be distinct, incomplete and complementary - this is an aspect of the basic structure of reality.
But Mormon complementarity does not sit comfortably with any mainstream Christian metaphysics, which has it that each soul is both saved and undergoes theosis as an individual unit - i.e. for classical theology the unit of humanity is a unit - and which therefore regards the dyad of marriage as a temporary earthly expedient which is necessarily dissolved by death, and Heaven as a sex-less kind of place.
For classical Christian theology, no-sex and unsexed is regarded as a higher state of being than the distinction-between and eternally-sealed union of man and woman.
Therefore, in mainstream discourse there are only three possible relations of the sexes, the domination either of 1. men, or of 2. women, or else 3. absolute sexual equality; but Mormonism adds the fourth possibility of radical, ultimate, metaphysical, dyadic complementarity.