Catholics say homosexuality is against natural law because God created the sexual faculty for procreation between male and female. What about those animals that have the choice of both sexes but prefer the same sex?
Natural law pertains to creatures who can reason. The Old Catholic Encyclopedia quotes St. Thomas Aquinas when defining natural law: "According to St. Thomas, the natural law is ‘nothing else than the rational creature’s participation in the eternal law’ (I-II, Q. xciv)."
Pope John Paul II, in his encyclical Veritatis Splendor (The Splendor of Truth), said that the natural law of moral theology "receives this name not because it refers to the nature of irrational beings, but because the reason which promulgates it is proper to human nature" (Veritatis Splendor 42). The moral law is called "natural law" because it is based on our nature as rational beings. It is not based on the nature of irrational beings, such as animals, plants, or inanimate matter.