The sins that we must avoid do not always manifest themselves outwardly.  Sins of the heart are condemning as well.  The truth is, outward sins always originate in the heart.

Matthew 15:19 (ESV)
¹⁹  For out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false witness, slander.

Before the outward sins that Jesus refers to are committed, they are thought about in the heart.  This is probably why Jesus mentioned evil thoughts first in this list.  Evil thoughts lead to murder, sexual immorality, slander, etc.  This is what Jesus was warning about in the sermon on the mount regarding adultery.  

Matthew 5:27-28 (ESV)
²⁷  “You have heard that it was said, ‘You shall not commit adultery.’ ²⁸  But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart.

Some have erroneously concluded from this verse that when a spouse does something like view pornography, it is grounds for divorce.  No, only the act of adultery is grounds for divorce (Matthew 5:32; 19:9), not the thought of it.  Jesus is not saying that this person has actually committed adultery, but that he has thought about it.  He has purposely imagined it in his heart.  The NKJV says, “…whoever looks at a woman to lust for her…”  That is the very purpose of looking at her, and that lustful look will condemn him just as much as the physical act.  In fact, at that point, the only thing stopping him from committing the act is the opportunity.

Lusting for anything is condemned by God.  The Greek word for lust is used for both harmless desire and things for which we covet.  When the word is translated as lust, it always denotes something sinful.  Whether we are having lustful thoughts for others or material things, it condemns us the same.  Our hearts must be kept pure.

Matthew 5:8 (ESV)
⁸  “Blessed are the pure in heart, for they shall see God.

Our souls were purified when we obeyed the gospel (1 Peter 1:22), but we must keep ourselves pure.  We can do that by guarding what we allow to penetrate our hearts through the gateway of our eyes.  Be careful what you watch because it can have a corrupting influence on your mind.  Instead of filling your mind with the immoral things of this world, fill it with the Word of God.  It will help you avoid lustful thoughts.

Psalm 12:6 (ESV)
⁶  The words of the LORD are pure words, like silver refined in a furnace on the ground, purified seven times.

Remember, it is only the pure in heart that will see God.

Psalm 24:3-4 (ESV)
³  Who shall ascend the hill of the LORD? And who shall stand in his holy place? ⁴  He who has clean hands and a pure heart…”  

As you wind down for the night, think about these things.