D&C 63:16

Those who lust in their heart or mind have committed adultery. And those who do so will not have the Spirit to be with them but it will be void in their lives. Those who don't even commit adultery who have lustful thoughts will still not have the Spirit with them. For all emotion takes over the mind if the thoughts are allowed to persist, and the Spirit is completely shut out. So let our minds and hearts be centered around God.