Marriage is considered holy mainly, because God Himself established it already there in the Garden of Eden giving it as a gift unto the first human pair. "Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh" (Gen. 2:24). Then, as well as now, the Word of God joins the couple.
Further, Jesus confirmed it in the New Testament with the following words, "Wherefore they are no more twain, but one flesh. What therefore, God hath joined together, let not man put asunder" (Matt. 19:5).
The holiness of marriage is independent of the couple or officiant. It is holy solely because God has established it with His Word.
By nature that which is holy is dedicated to the service of God. Herein lie great wisdom and a solid foundation for married life. God has intended that marriage would serve Him. When understood correctly both husband and wife labor to keep it holy by living in it, according to His Word. All other aspects of married life fall into place when the marriage is dedicated unto the service of God and to help fulfill His will.
Courtesy by Zac Poonen
(Till you Marry)