There is an element of sinfulness that enters into every human thought and desire, a sinfulness that is rooted in the fall. (See the ATQ article How Can Christians Believe that the Human Race Is Depraved?) In a sense, no human desire is untainted with sin. Because an element of evil intrudes into every human interaction and relationship, it is impossible for human beings to achieve absolute purity.
Along with our depravity, our composite physical nature is a factor. Like all animals, humans are physical beings with instinctual sexual/mating desires “hardwired” into us. Because of our instinctual mating desires, it is normal for people to struggle to suppress inappropriate sexual thoughts and feelings towards attractive people of the opposite sex. This is natural.
The Bible makes it clear that although temptation is the result of sin in the sense of it being an aspect of our fallen world, mere temptation isn’t something that God holds us accountable for. Even Jesus experienced temptation (Hebrews 4:15). But though we aren’t accountable for the temptations we experience, the Lord made it clear that we are accountable for sinful responses to temptation—whether it be preferential treatment of the individual or conscious lust (Matthew 5:28-30).
The moment someone consciously sexualizes his admiration of another person’s beauty—transforming admiration into lust—he commits soul-damaging sin that warrants judgment. The willful cultivation of wrongfully sexualized thoughts arouses further destructive sexual feelings that wreak havoc in a person’s spiritual, emotional, and relational life. Sinful responses to instinctive sexual desires increase the power of temptation and result in enslavement to ugly, compulsive behavior.