We further Declare that God has Commanded that the Sacred Powers of Procreation are to be Employed only between Man and Woman, Lawfully Wedded as Husband and Wife.

Physical intimacy between husband and wife is beautiful and sacred. It is ordained of God for the creation of children and for the expression of love within marriage. In the world today, Satan has led many people to believe that sexual intimacy outside of marriage is acceptable. But in God's sight, it is a serious sin. It is an abuse of the power He has given us to create life. The prophet Alma taught that sexual sins are more serious than any other sins except murder and denying the Holy Ghost (see Alma 39:3-5). Sometimes people try to convince themselves that sexual relations outside of marriage are acceptable if the participants love one another. This is not true. Breaking the law of chastity and encouraging someone else to do so is not an expression of love. People who love each other will never endanger one another's happiness and safety in exchange for temporary personal pleasure. When people care for one another enough to keep the law of chastity, their love, trust, and commitment increase, resulting in greater happiness and unity. In contrast, relationships built on sexual immorality sour quickly. Those who engage in sexual immorality often feel fear, guilt, and shame. Bitterness, jealousy, and hatred soon replace any positive feelings that once existed in their relationship. Our Heavenly Father has given us the law of chastity for our protection. Obedience to this law is essential to personal peace and strength of character and to happiness in the home. Those who keep themselves sexually pure will avoid the spiritual and emotional damage that always comes from sharing physical intimacies with someone outside of marriage. Those who keep themselves sexually pure will be sensitive to the Holy Ghost's guidance, strength, comfort, and protection and will fulfill an important requirement for receiving a temple recommend and participating in temple ordinances. -See True to the Faith (2004), 29-33


Jeffrey R. Holland, BYU Devotional

Scriptures:

Thou shalt not commit adultery. -Exodus 20:14, Holy Bible

Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body. What? know ye not that your body is the temple of the Holy Ghost which is in you, which ye have of God, and ye are not your own? For ye are bought with a price: therefore glorify God in your body, and in your spirit, which are God’s. -1 Corinthians 6: 18-19, Holy Bible

Behold, it is written by them of old time, that thou shalt not commit adultery; But I say unto you, that whosoever looketh on a woman, to lust after her, hath committed adultery already in his heart. Behold, I give unto you a commandment, that ye suffer none of these things to enter into your heart; For it is better that ye should deny yourselves of these things, wherein ye will take up your cross, than that ye should be cast into hell. -3 Nephi 12:27-30, Book of Mormon

Thou shalt love thy wife with all thy heart, and shalt cleave unto her and none else. And he that looketh upon a woman to lust after her shall deny the faith, and shall not have the Spirit; and if he repents not he shall be cast out. Thou shalt not commit adultery; and he that committeth adultery, and repenteth not, shall be cast out. But he that has committed adultery and repents with all his heart, and forsaketh it, and doeth it no more, thou shalt forgive; But if he doeth it again, he shall not be forgiven, but shall be cast out. -Doctirine & Covenants 42: 22-26

Wherefore God also gave them up to uncleanness through the lusts of their own hearts, to dishonour their own bodies between themselves: Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen. For this cause God gave them up unto vile affections: for even their women did change the natural use into that which is against nature: And likewise also the men, leaving the natural use of the woman, burned in their lust one toward another; men with men working that which is unseemly, and receiving in themselves that recompence of their error which was meet. And even as they did not like to retain God in their knowledge, God gave them over to a reprobate mind, to do those things which are not convenient; Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers, Backbiters, haters of God, despiteful, proud, boasters, inventors of evil things, disobedient to parents, Without understanding, covenantbreakers, without natural affection, implacable, unmerciful: Who knowing the judgment of God, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them. -Romans 1:24-32, Holy Bible

Definitions:

Powers - ability to do or act; capability of doing or accomplishing something.
Procreation - to beget or generate (offspring); to produce; bring into being.
Employed - to make use of (an instrument, means, etc.); use; apply; to occupy or devote (time, energies, etc.).
Lawfully - recognized or sanctioned by law; legitimate; acting or living according to the law.
Wedded - united in matrimony; married.
-Dictionary.com