It would be superfluous to receive by faith, things that can be known by natural reason
Thomas AquinasThe meaning of what is said is according to the motive for saying it: because things are not subject to speech, but speech to things. Therefore we should take account of the motive of the lawgiver, rather than of his very words.
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