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.
Elder M. Russell Ballard (Gen Conf Apr 1993)

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). Elder Boyd K. Packer, Apr Conf 1992