City of God

It is this which those canine or cynic philosophers have overlooked, when they have, in violation of the modest instincts of men, boastfully proclaimed their unclean and shameless opinion, worthy indeed of dogs, viz., that as the matrimonial act is legitimate, no one should be ashamed to perform it openly, in the street or in any public place. Instinctive shame has overborne this wild fancy. For though it is related that Diogenes once dared to put his opinion in practice, under the impression that his sect would be all the more famous if his egregious shamelessness were deeply graven in the memory of mankind, yet this example was not afterwards followed. Shame had more influence with them, to make them blush before men, than error to make them affect a resemblance to dogs.

And possibly, even in the case of Diogenes, and those who did imitate him, there was but an appearance and pretence of copulation, and not the reality. Even at this day there are still Cynic philosophers to be seen; for these are Cynics who are not content with being clad in the pallium, but also carry a club; yet no one of them dares to do this that we speak of. If they did, they would be spat upon, not to say stoned, by the mob. Human nature, then, is without doubt ashamed of this lust; and justly so, for the insubordination of these members, and their defiance of the will, are the clear testimony of the punishment of man's first sin.

And it was fitting that this should appear specially in those parts by which is generated that nature which has been altered for the worse by that first and great sin—that sin from whose evil connection no one can escape, unless God's grace expiate in him individually that which was perpetrated to the destruction of all in common, when all were in one man, and which was avenged by God's justice.