Why do Church leaders place so much emphasis on avoiding sexual sin?

Elder Jeffrey R. Holland of the Quorum of the Twelve Apostles has offered three reasons for the emphasis on sexual purity:

“First is the revealed, restored doctrine of the human soul.

“… ‘The spirit and the body are the soul of man’ [D&C 88:15] and … when the spirit and body are separated, men and women ‘cannot receive a fulness of joy’ [D&C 93:34]. …

“… One who uses the God-given body of another without divine sanction abuses the very soul of that individual, abuses the central purpose and processes of life. …

“Secondly, … human intimacy is reserved for a married [man and woman] because it is the ultimate symbol of total union, a totality and a union ordained and defined by God. …

“In matters of human intimacy, you must wait! … If you persist in pursuing physical satisfaction without the sanction of heaven, you run the terrible risk of such spiritual, psychic damage that you may undermine both your longing for physical intimacy and your ability to give wholehearted devotion to a later, truer love. …

“Thirdly, … physical intimacy is not only a symbolic union between a husband and a wife … but it is also symbolic of a shared relationship between them and their Father in Heaven” (“Personal Purity,” Ensign, Nov. 1998, 76–77).