The vice of lust is the desire for immoral or sinful sexual pleasure. Having a desire for sex is normal and human. Controlling it is also human, since men and women have a free will and rational intellect.
Animals have sex because of instinct. Human beings freely choose to have sex and thus, as a conscious and deliberate act of the free will, it is a moral act.
Human sexuality is not intrinsically evil, and when it occurs between a husband and wife and is oriented to love (unity) and life (procreation), it is a holy and sacred act of marriage. When human sex occurs outside or before marriage, with members of the same sex or by oneself, it is sinful and immoral, as it is an abuse of a gift reserved for those in marriage.
Jesus said in the Gospel, “You have heard that it was said, ‘You shall not commit adultery.’ But I say to you, everyone who looks at a woman with lust has already committed adultery with her in his heart” (Matthew 5:27). The Church interprets this to mean that the sixth Commandment forbids not only acts of adultery, but also willful and deliberate thoughts as well.
Sin can be committed in thought, word, or deed. Lust is the intentional and conscious desire to have immoral sexual pleasure. Involuntary and spontaneous impure thoughts that plague everyone at some time or another after puberty are not subjectively sinful since there is no deliberate act of the free will.
Intentionally arousing dirty thoughts or cultivating sexual fantasies while fully awake is considered sinful. Pornography and frequenting strip clubs are also deemed immoral, as they divorce sexual intimacy from the sacredness of marriage and degrade the person being looked at by presenting him or her as a sex object—not as a person made in the image and likeness of God.