Before Eve ate of the forbidden tree she "desired" it because it could make her wise:
"And when the woman saw that the tree was good for food, and that it was pleasant to the eyes, and a tree to be desired to make one wise, she took of the fruit thereof, and did eat" (Gen.3:6).
She had this "desire" for the tree before she partook of it so she had a desire for that which was forbidden. What is the difference between that and what the Lord Jesus spoke of here?:
"But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart" (Matthew 5:28).
The Greek word translated "lust after" means "to have a desire for" (Thayer's Greek English Lexicon).
Secondly, are you saying that 'infants" desire forbidden things? If your answer is "no" then you have not yet demonstrated that sin dwells in infants.
As you say, lust simply means desire. Desires can either be good or bad.
Eve desired the fruit of the tree of the knowledge of good and evil. But the Bible says she was deceived. Remember she was created perfect, in perfect innocence and had not known "evil" up to this point. It was Satan that tempted her, and she gave into this beautiful looking fruit, "that was pleasant to the eyes," and only according to Satan "able to make her wise." She had already been deceived, so deceived that she was convinced that wisdom would come from eating of the tree than from trusting in God. She had been listening to Satan too long.
See the difference. When a man lusts after a woman, he doesn't have to. First, he has the law of God already written in his heart. His conscience tells him it is wrong. He has already tasted evil. That is something Eve had never known. He absolutely knows right from wrong. He chooses to lust after something that is not his to have, and if he takes that which is not his to have, he commits more than one sin: lust, adultery, (and whatever else may be involved--kidnapping? molestation, anger, rape, assault, etc.) One sin leads to another and another. But the first sin was committed in the heart--the lust or desire to have something that was not his--coveting and lusting for someone else's wife or daughter.
Sin dwells in a person who is capable of thinking or reasoning--old enough to choose between right and wrong.
The sin nature dwells in all mankind, infants included. It is inherited. It is called the Adamic nature for it comes from Adam. It is part of the curse. The entire creation is under a curse and groans and travails even now. We wait for the redemption of our bodies, the Bible says.