It is written:
Matthew 5:28-30 But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart. And if thy right eye offend thee, pluck it out, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell. And if thy right hand offend thee, cut it off, and cast it from thee: for it is profitable for thee that one of thy members should perish, and not that thy whole body should be cast into hell.
That is Jesus talking in the above passage. Also consider:
1 John 2:15-17 Love not the world, neither the things that are in the world. If any man love the world, the love of the Father is not in him. For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world. And the world passeth away, and the lust thereof: but he that doeth the will of God abideth for ever.
When Jesus says we've already committed adultery in the heart by looking upon someone with lust, we should repent/confess and get under the blood of Jesus and get right with the Master, for He's soon to return. You may ask about the heart and it's matters:
1 Samuel 16:7 But the LORD said unto Samuel, Look not on his countenance, or on the height of his stature; because I have refused him: for the LORD seeth not as man seeth; for man looketh on the outward appearance, but the LORD looketh on the heart.
Jeremiah 17:9-10 The heart is deceitful above all things, and desperately wicked: who can know it? I the LORD search the heart, I try the reins, even to give every man according to his ways, and according to the fruit of his doings.
Mark 7:20-23 And he said, That which cometh out of the man, that defileth the man. For from within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, Thefts, covetousness, wickedness, deceit, lasciviousness, an evil eye, blasphemy, pride, foolishness: All these evil things come from within, and defile the man.
Regarding temptation:
Matthew 26:41 Watch and pray, that ye enter not into temptation: the spirit indeed is willing, but the flesh is weak.
James 1:12-16 Blessed is the man that endureth temptation: for when he is tried, he shall receive the crown of life, which the Lord hath promised to them that love him. Let no man say when he is tempted, I am tempted of God: for God cannot be tempted with evil, neither tempteth he any man: But every man is tempted, when he is drawn away of his own lust, and enticed. Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death. Do not err, my beloved brethren.
Yes we are just dust (Psalms 103:14) and that we're always mindful of God (Psalms 8:4). God's mercies are NEW every morning (Lamentations 3:22-23). When we confess and TRULY turn away (repent) of our sin, God is faithful and forgives us:
1 John 1:9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
If it's a stronghold, generational curse or weakness then confess/repent each time you do it. According to what Jesus tells us it is likewise wrong to fantacize, imagine, meditate upon sin and sinful behaviors. We are to cast down these imaginations and thoughts that lead to sin:
2 Corinthians 10:3-6 For though we walk in the flesh, we do not war after the flesh: (For the weapons of our warfare are not carnal, but mighty through God to the pulling down of strong holds;) Casting down imaginations, and every high thing that exalteth itself against the knowledge of God, and bringing into captivity every thought to the obedience of Christ; And having in a readiness to revenge all disobedience, when your obedience is fulfilled.
It is written:
Proverbs 23:7 For as he thinketh in his heart, so is he: Eat and drink, saith he to thee; but his heart is not with thee.
Those who tolerate sin and are in leadership positions regarding Christianity WILL BE JUDGED for their actions Luke 12:47-48 + James 3:1):
Ezekiel 3:16-21 And it came to pass at the end of seven days, that the word of the LORD came unto me, saying, Son of man, I have made thee a watchman unto the house of Israel: therefore hear the word at my mouth, and give them warning from me. When I say unto the wicked, Thou shalt surely die; and thou givest him not warning, nor speakest to warn the wicked from his wicked way, to save his life; the same wicked man shall die in his iniquity; but his blood will I require at thine hand. Yet if thou warn the wicked, and he turn not from his wickedness, nor from his wicked way, he shall die in his iniquity; but thou hast delivered thy soul. Again, When a righteous man doth turn from his righteousness, and commit iniquity, and I lay a stumblingblock before him, he shall die: because thou hast not given him warning, he shall die in his sin, and his righteousness which he hath done shall not be remembered; but his blood will I require at thine hand. Nevertheless if thou warn the righteous man, that the righteous sin not, and he doth not sin, he shall surely live, because he is warned; also thou hast delivered thy soul.
Romans 14:12 So then every one of us shall give account of himself to God.
Remember NOT everyone will go to Heaven (Matthew 7:21-23):
Romans 8:29-30 For whom he did foreknow, he also did predestinate to be conformed to the image of his Son, that he might be the firstborn among many brethren. Moreover whom he did predestinate, them he also called: and whom he called, them he also justified: and whom he justified, them he also glorified.
The source of this message is God (II Timothy 3:16). Also another good chapter to study in is Ezekiel 18. It tells in further detail about these seemingly "sticky" things that we struggle with on the earth. Yet, the Word of God is VERY clear to NOT tolerate ANY kind of sexual sin. It destroys and pollutes the soul, destroys your witness for Christ, causes the anointing to NOT dwell in an unclean temple (I Corinthians 6:15-20). Any sexual sin is a LUSTFUL behavior and we should RUN from it:
1 Corinthians 6:18 Flee fornication. Every sin that a man doeth is without the body; but he that committeth fornication sinneth against his own body.
Be blessed!