2. It Violates Natural Law
Marriage is not just any relationship between human beings. It is a relationship rooted in human nature and thus governed by natural law.
Natural law’s most elementary precept is that “good is to be done and pursued, and evil is to be avoided.” By his natural reason, man can perceive what is morally good or bad for him. Thus, he can know the end or purpose of each of his acts and how it is morally wrong to transform the means that help him accomplish an act into the act’s purpose.
Any situation which institutionalizes the circumvention of the purpose of the sexual act violates natural law and the objective norm of morality.
Being rooted in human nature, natural law is universal and immutable. It applies to the entire human race, equally. It commands and forbids consistently, everywhere and always. Saint Paul taught in the Epistle to the Romans that the natural law is inscribed on the heart of every man. (Rom. 2:14-15)