Unity involves an "ever perfecting" action, an ever fuller cooperation with God.

Real marriage requires what's called a conjugal union, a totality of union of the body, the mind, the soul, the will, the intellect. 

God gave males and females different capacities - those are written on and within the body.

Some of these "capacities" make no sense by themselves; they only make sense when one looks at the complementariness of capacities of the other gender.

These capacities or faculties of course aren't just at the organ level; they are also at the hormonal, cellular, and DNA level.  Complex complementarity. 

Love must be open to grow beyond itself, to reach an every higher state of perfection and participation in the life of God, which includes creation.  God can't help Himself from creating. He is always creating. 

A husband and wife must be open and configured to this enlarging of creation, a participation in the life of God.