With all due respect, Mike, your reply to the OP sheds little light on the question. In fact, the “desirelessness” of which you speak is something we cannot achieve until we are fully united with God in heaven. Even the most faithful saint has had to fight an ongoing battle against temptation. With faith and perseverance, one can strengthen one’s will against temptation, but weakened desires are desires nonetheless.
OP, lust is a perversion of chastity. It is normal to feel attractions, and this is part of what leads us to choose the spouse we choose. Lust, however, is selfish; where love and chastity in a marriage involve giving of self, lust involves taking and using.
Fantasizing about someone – even one’s spouse – is acting on lustful urges. Sex is supposed to unite the spouses; fantasizing is just one person “getting off” to some degree. Desiring one’s spouse for the sake of “getting off” is sinful – being sexually attracted to him/her is not.
Lustful thoughts that pop into one’s mind out of habit are not sinful unless one freely chooses to dwell on or act on them. Eventually, one can reconditon one’s will (with the help of God’s grace) so that such thoughts do not pop in anymore – or, at least, do so infrequently and do not cause one as much temptation.
Peace,
Dante