The highest manifestation of life consists in this: that a being governs its own actions. A thing which is always subject to the direction of another is somewhat of a dead thing.
Thomas AquinasSecondly, man sins against nature when he goes against his generic nature, that is to say, his animal nature. Now, it is evident that, in accord with natural order, the union of the sexes among animals is ordered towards conception. From this it follows that every sexual intercourse that cannot lead to conception is opposed to man's animal nature.
Thomas Aquinas