The magnitude of the punishment matches the magnitude of the sin. Now a sin that is against God is infinite; the higher the person against whom it is committed, the graver the sin-it is more criminal to strike a head of state than a private citizen-and God is of infinite greatness. Therefore an infinite punishment is deserved for a sin committed against Him.
Thomas AquinasWe are like children, who stand in need of masters to enlighten us and direct us; God has provided for this, by appointing his angels to be our teachers and guides.
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