What determines the morality of an act? In moral theology, an act is the knowing choice of a human person. Each knowing choice is an act, and each act is subject to the eternal moral law. Some acts are moral, and other acts are immoral. An immoral act is a sinful act.
Sex is intended to be both an expression of love for your spouse, and a beautiful means of procreation. Sex is so special, powerful, and valuable that it is properly used only within marriage. Sex is the ultimate gift husbands and wives can give: a total gift of self, body and soul.
Note from Ville Hietanen Jerome : I do no longer adhere to the position that masturbation is a mortal sin against the Natural Law in the sense that any lawbreaker would automatically be committing a mortal sin and be damned if he or she died unrepentant without a will to confess or feeling sorrow from simply committing this act - even if he or she did not know or understand this act was sinful - as currently expressed in this article. Now I follow the Law of Conscience as expressed by St. Alphonsus in his Moral Theology , which teaches us that no one will be committing a sin unless they beforehand knew about that the act they committed was unlawful. So does this mean all acts are not sinful unless we know they are sinful beforehand, such as murder?