What is lust and why is it sin?
Technically lust is defined as an intense desire for something or someone. How is one to measure the degree of desire and at what point does a desire become intense enough to be labelled sin?
First, desires require an object. It is impossible to desire nothingness. This is the basic inconsistency with Buddhism in its purest sense. Desires will always exist and they are fixed upon an object. Whether it is a woman, a car, a job, a status or a Tag Heuer watch, people desire objects. That object must be a specific person or thing, whether they are before your eyes, or on the pages of a magazine. One of the Ten Commandments is "You shall not covet your neighbours' house, wife, possessions ..." Inherit in this covetousness is desire for something.
Second, God commands "do not love the world or anything in the world" (1 John). What did he mean? Shall we not love our families, puppies, Christmas dinners and the budding flowers in spring. Answer: No, we shouldn't. Not if the love of these things are an end in themselves. Any love and desire for anything that does not ultimately leads us to a love and desire for the glory of God is sin. Sin is when we exchange the glory of God for ANYTHING else - however warm and fuzzy it might be.
Third, the lusting after a person occurs when your desire is turned towards yourself. If you are attracted to someone or something, praise God - you're normal. When you appreciate physical beauty or charming characteristics, praise God - He's a creative and wonderful God. But as soon as desire does not translate to love for God or the others, the only person left to receive the benefits is yourself. We are commanded, "Love the Lord and love one another." The sin of Adam and Eve was a sin of lust: a lust for nourshment, independence and power. Everyone who ever committed the sin of lust thought of how they might take and use that object for their own personal gain.
Fourth, lust is adultery. In the sexual sense, the lust that is conceived in the mind of man is fully evident before the eyes of God. You think it, you did it. Spiritually, lust is adultery of the idolatrous kind. Adultery is unfaithfulness and a prostituting of oneself to someone to whom you are not joined. Lust diminishes the supremacy of God, casting Him momentarily aside.