15 The whole stress, then, of this question comes to this; whether it be true universally that no sin of another, committed upon you, is to be imputed to you, if, being able to avoid it by a lighter sin of your own, you do it not; or whether there be an exception of all bodily defilement. No man says that a person is defiled by being murdered, or cast into prison, or bound in chains, or scourged, or afflicted with other tortures and pains, or proscribed and made to suffer most grievous losses even to utter nakedness, or stripped of honors, and subjected to great disgrace by reproaches of whatsoever kind; whatever of all these a man may have unjustly suffered, no man is so senseless as to say that he is thereby defiled.
But if he have filth poured all over him, or poured into his mouth, or crammed into him, or if he be carnally used like a woman; then almost all men regard him with a feeling of horror, and they call him defiled and unclean. One must conclude then that the sins of others, be they what they may, those always excepted which defile him on whom they are committed, a man must not seek to avoid by sin of his own, either for himself or for any other, but rather he must put up with them, and suffer bravely; and if by no sins of his own he ought to avoid them, therefore not by a lie: but those which by being committed upon a man do make him unclean, these we are bound to avoid even by sinning ourselves; and for this reason those things are not to be called sins, which are done for the purpose of avoiding that uncleanness.
For whatever is done, in consideration that the not doing it were just cause of blame, that thing is not sin. Upon the same principle, neither is that to be called uncleanness when there is no way of avoiding it; for even in that extremity he who suffers it has what he may do aright, namely, patiently bear what he cannot avoid. Now no man while acting aright can be defiled by any corporal contagion. For the unclean in the sight of God is every one who is unrighteous; clean therefore is every one who is righteous; if not in the sight of men, yet in the sight of God, Who judges without error.
Nay, even in the act of suffering that defilement with power given of avoiding it, it is not by the mere contact that the man is defiled; but by the sin of refusing to avoid it when he might. For that would be no sin, whatever might be done for the avoiding of it. Whoever therefore, for the avoiding of it, shall tell a lie, sins not.
Source: On Lying (New Advent)