When we do workshops we often give participants a test. Define marriage, but don’t use any religious language.
They can’t do it.
Well, it’s a trick question. Marriage can’t be defined. It is a relationship that is an integral part of God’s plan for creation which can’t be defined, only describe. Check it out.
In Scripture it is described as the one-flesh union; In the Catechism as “the intimate community of life and love. Saint John Paul II described it as a communion of man and woman leading to the communion of parent and child, and communion of generations. Pope Benedict described it as an icon of the Holy Trinity and like the Trinity is life giving.
We often try to describe marriage by what is essential to it – complementary of spouses, potential procreators, the conjugal act, and so on, but that goes over people’s heads because none of those things are particular to marriage.
Today the fullness of marriage can only be known by what it does which is also particular to it – the fact that it unites children with their mothers and fathers. Not all married men and women have children, but every child has a mother and father.
Remember and use this description: Marriage unites a man and a woman with each other and any children born from their union. Or put more simply, the reality of marriage is the only institution that unties children with their own mother and father.
 Catechism of the Catholic Church §1603