Fact: Sacred means to be set apart for the service of God.
How can we be of service to God? What is it that he most desires? In the words of the Baltimore Catechism, God created us to know him, to love him, and to serve him on earth and to be happy with him in heaven.
There are many ways to set ourselves apart to achieve God’s will in this regard. Marriage is one of them.
Marriage is the union of one man and one woman in an exclusive, life long relationship for the purpose of procreation and the education of children and for uniting husband and wife in such a way that they may remain faithful to their marital vows for the ultimate good of the children they rear and for the salvation of their own souls; therefore, marriage is sacred, because it is a relationship set apart to achieve the end for which human beings were created.
For a good book on the subject I suggest, “Marriage: The Rock on Which the Family is Built” by William E. May and published by Ignatius Press.