Christianity and Judaism
In contrast to the Jewish view of being morally balanced, Original Sin refers to the idea that the sin of Adam and Eve's disobedience (sin "at the origin") has passed on a spiritual heritage, so to speak. Christians teach that human beings inherit a corrupted or damaged human nature in which the tendency to do bad is greater than it would have been otherwise, so much so that human nature would not be capable now of participating in the afterlife with God. This is not a matter of being "guilty" of anything; each person is only personally guilty of their own actual sins. However, this understanding of original sin is what lies behind the Christian emphasis on the need for spiritual salvation from a spiritual Saviour, who can forgive and set aside sin even though humans are not inherently pure and worthy of such salvation. Paul the Apostle in Romans and I Corinthians placed special emphasis on this doctrine, and stressed that belief in Jesus would allow Christians to overcome death and attain salvation in the hereafter.
Lust
St. Thomas Aquinas defines the sin of Lust in questions 153 and 154 of his Summa Theologica. Aquinas says the sin of lust is of "voluptuous emotions," and makes the point that sexual pleasures, "unloosen the human spirit," and set aside right reason (Pg.191). Aquinas restricts lust's subject matter to physical desires specifically arising from sexual acts, but Aquinas does not assume all sex-acts are sinful. Sex is not a sin in marriage, because sex is the only way for humans to reproduce. If sex is used naturally and the end purpose is reproduction there is no sin. Aquinas says, "if the end be good and if what is done is well-adapted to that, then no sin is present," (Pg.193). However, sex simply for the sake of pleasure is lustful and therefore, a sin. A man who uses his body for lechery wrongs the Lord.