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. (D&C 42:22-23.)
“And verily I say unto you, as I have said before, he that looketh on a woman to lust after her, or if any shall commit adultery in their hearts, they shall not have the Spirit, but shall deny the faith and shall fear” (D&C 63:16)