I have a small problem with this. Man is naturally fallible, but I don't believe that man is naturally evil. I think evil is a choice.
Mat 5:28 But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
a·dul·ter·y n. pl. a·dul·ter·ies
Voluntary sexual intercourse between a married person and a partner other than the lawful spouse.
This makes it fairly clear. If you are married and you think about someone else sexually, you are committing adultery in your heart. Likewise, if you think about someone who is married to someone else in a sexual manner, you are also committing adultery and coveting someone else's husband/wife.
However, where does the Bible say that sexual desire in itself is evil?