In the Beginning... God created this immense expanse of the cosmos, and very particularly the earth, and He filled it with all sorts of delightful vegetation and animal life - and all was Good. But then He took some dust and fiddled around with it, and when He had breathed into it, He had created Man.
Now, Man is not a gender, but a species. "God created man. Male and female He created them." Male and female. Two genders, both reflecting in their bodies and in their unique natures, masculine and feminine, the Personality of the Creator.
It is very profound that God has chosen to give us a body - that physical part of us that is so often an embarrassment and a discomfort, a source of suffering and a reminder of our severe limitations. The body becomes not only the house of our own soul, but it is also the Temple of the Holy Spirit (I Cor 6:19).
Moreover, when we marry, and our bodies are united in the act of marriage (sexual intercourse) St. Paul says that we do become one flesh! and our sexual union with our spouse proclaims the "great mystery" of Christ and the Church (Eph. 5:31-32)
In being created male and female, God has given us a visible physical complementarity - the man's straight linear body shape to our various curves; the corresponding angles of the genitals for perfect union in the marital embrace only further signify the mystery of our complementarity.
But our complementarity goes beyond the physical. The perplexing psychological and spiritual differences between men and women have boggled thinkers and theorists for millenia! and, in all likelihood, will continue to do so til the end of life on Earth.
So this business of marriage becomes ennobled - not as the historical, practical means of securing lands and properties, titles, political allegiances, and even more mundane considerations of having a woman to keep house and a man to provide some practical and social security and stability -
In marriage - in the very deliberate joining of two distinct and often-disparate lives, we become a Theology - a study and revelation in the Nature and Person-ality of God!
For Catholics, a sacrament has two parts: a verbal and a physical. In our Communion, for example, the words of the Consecration are united with the elements of bread and wine to form a Sacrament; in Confirmation, the words are united with chrism oil. But in marriage - the words are the vows exchanged at the time of the wedding - and the physical element is their very own bodies, freely given in the consummation of the marriage on the wedding night!