Saint Thomas Aquinas:
"Given 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…
"If all the sins of the flesh are worthy of condemnation because by them man allows himself to be dominated by that which he has of the animal nature, much more deserving of condemnation are the sins against nature by which man degrades his own animal nature…
"Man can sin against nature in two ways. First, when he sins against his specific rational nature, acting contrary to reason. In this sense, we can say that every sin is a sin against man’s nature, because it is against man’s right reason…
“Secondly, 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.”
Saint Peter Damian:
"Truly, this vice is never to be compared with any other vice because it surpasses the enormity of all vices.… It defiles everything, stains everything, pollutes everything. And as for itself, it permits nothing pure, nothing clean, nothing other than filth.…
"The miserable flesh burns with the heat of lust; the cold mind trembles with the rancor of suspicion; and in the heart of the miserable man chaos boils like Tartarus [Hell]…. In fact, after this most poisonous serpent once sinks its fangs into the unhappy soul, sense is snatched away, memory is borne off, the sharpness of the mind is obscured. It becomes unmindful of God and even forgetful of itself. This plague undermines the foundation of faith, weakens the strength of hope, destroys the bond of charity; it takes away justice, subverts fortitude, banishes temperance, blunts the keenness of prudence.
“And what more should I say since it expels the whole host of the virtues from the chamber of the human heart and introduces every barbarous vice as if the bolts of the doors were pulled out.”