From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
In this context [of the supposed opposition of nature and freedom], objections of physicalism and naturalism have been levelled against the traditional conception of the natural law, which is accused of presenting as moral laws what are in themselves mere biological laws. Consequently, in too superficial a way, a permanent and unchanging character would be attributed to certain kinds of human behaviour, and, on the basis of this, an attempt would be made to formulate universally valid moral norms. According to certain theologians, this kind of “biologistic or naturalistic argumentation” would even be present in certain documents of the Church’s Magisterium, particularly those dealing with the area of sexual and conjugal ethics. It was, they maintain, on the basis of a naturalistic understanding of the sexual act that contraception, direct sterilization, autoeroticism, pre-marital sexual relations, homosexual relations and artificial insemination were condemned as morally unacceptable. In the opinion of these same theologians, a morally negative evaluation of such acts fails to take into adequate consideration both man’s character as a rational and free being and the cultural conditioning of all moral norms. In their view, man, as a rational being, not only can but actually must freely determine the meaning of his behaviour. This process of “determining the meaning” would obviously have to take into account the many limitations of the human being, as existing in a body and in history. Furthermore, it would have to take into consideration the behavioural models and the meanings which the latter acquire in any given culture. Above all, it would have to respect the fundamental commandment of love of God and neighbour. Still, they continue, God made man as a rationally free being; he left him “in the power of his own counsel” and he expects him to shape his life in a personal and rational way. Love of neighbour would mean above all and even exclusively respect for his freedom to make his own decisions. The workings of typically human behaviour, as well as the so-called “natural inclinations”, would establish at the most — so they say — a general orientation towards correct behaviour, but they cannot determine the moral assessment of individual human acts, so complex from the viewpoint of situations.
— Veritatis Splendor 47