Elder M. Russell Ballard, Some young men and women in the Church talk openly about sexual transgression. They seem to forget that the Lord forbids all sexual relations before marriage, including petting, sex perversion of any kind, or preoccupation with sex in thought, speech, or action. Some youth foolishly rationalize that it is "no big deal" to sin now because they can always repent later when they want to go to the temple or on a mission. Anyone who does that is breaking promises made to God both in the premortal life and in the waters of baptism. The idea of sinning a little is self-deception. Sin is sin! Sin weakens you spiritually, and it always places the sinner at eternal risk. Choosing to sin, even with the intent to repent, is simply turning away from God and violating covenants.
(Gen Conf Apr 1993)

Elder Boyd K. Packer, The only legitimate employment of the power of procreation is between husband and wife, man and woman, who have been legally and lawfully married. Anything else violates the commandments of God. From Alma, "If ye speak against it, it matters not, for the word of God must be fulfilled" (Alma 5:58). Apr Conf 1992

Elder Richard G. Scott, "Do not take comfort in the fact that your transgressions are not known by others...our every act is seen by our Father in Heaven and His Beloved Son. They know everything about us. Excusing transgression with a cover-up may appear to fix the problem, but it does not. The tempter is intent on making public your most embarrassing acts at the most harmful time. Lies weave a pattern that is ever more confining and becomes a trap that Satan will spring to your detriment." Conf Report, Apr 1995

Pres. Marion G. Romney, "David, though highly favored of the Lord, yielded to temptation. His unchastity led to murder, and as a consequence, he lost his families and his exaltation." Conf Report, Apr 1979

Pres. Gordon B. Hinckley, "It is my observation that a young man and a young woman who violate the principles of morality soon end up hating one another." (Quoting John A. Widtsoe) Ensign, June 1996