“You have heard that it was said, ‘You shall not commit adultery’. But I say to you that everyone who looks at a women with lustful intent has already committed adultery with her in his heart.” – Matthew 5:27-28
I want to preface this post by saying that the desire for sex is a gift from God. It is gift to single men to drive them to mature, get a job, grow in Christ and pursue marriage with a godly woman, in a godly way. It is a gift to married men to encourage them to pursue intimacy with their wife; by intimacy I do not mean sex, I do mean an exclusively, close, personal relationship. The desire for sex is as natural and good as the drive to eat and sleep. However, unlike other natural desires, the desire for sex is to be filled only within the bounds of a lifelong, love-filled covenant—dedicated to sacrificially serving one’s spouse for the glory of God—otherwise known as marriage.
In these verses and the surrounding passage Jesus takes the liberty as the Word of God to interpret the word of God. Herein Jesus reveals to us that God is not merely concerned with outward (visible and vocal) conformity to a standard but inwardly (heart worship) conforming to the ultimate standard: to be perfect as our heavenly Father is perfect. To illustrate this Jesus takes a long standing command: not committing adultery, and reveals its intended purpose: not committing idolatry. Lust is adultery of the heart, because it takes the desire for sex (which is good) and 'aims' or focuses it on a woman to whom the man is not solely dedicated to through covenant. Ultimately lust is against God, because it exalts its intentions above the command of God that a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh.
 Within this exclusively, close, personal relationship spouses are to pursue each other intellectually, emotionally and physically. This relationship should be a deepening friendship, with each spouse repenting and forgiving sin, while honoring and loving the other.
 John 1:1-3
 Matthew 5:48
 Genesis 2:24