It is clear that he does not pray, who, far from uplifting himself to God, requires that God shall lower Himself to him, and who resorts to prayer not to stir the man in us to will what God wills, but only to persuade God to will what the man in us wills.
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 AquinasGiven the sin of impiety through which they [the Romans] sinned against the divine nature [by idolatry], the punishment that led them to sin against their own nature followed.... I say, therefore, that since they changed into lies [by idolatry] the truth about God, He brought them to ignominious passions, that is, to sins against nature; not that God led them to evil, but only that he abandoned them to evil.
Thomas Aquinas