Chapter 18.
77 To have full knowledge of what is foul is in the highest degree shameful. To see anything of this sort, if by chance it should happen, how dreadful that is! What, therefore, is displeasing to us in others, can that be pleasing in ourselves? Is not nature herself our teacher, who has formed to perfection every part of our body, so as to provide for what is necessary and to beautify and grace its form? However she has left plain and open to the sight those parts which are beautiful to look upon; among which, the head, set as it were above all, and the pleasant lines of the figure, and the appearance of the face are prominent, while their usefulness for work is ready to hand. But those parts in which there is a compliance with the necessities of nature, she has partly put away and hidden in the body itself, lest they should present a disgusting appearance, and partly, too, she has taught and persuaded us to cover them.
78. Is not nature herself then a teacher of modesty? Following her example, the modesty of men, which I suppose is so called from the mode of knowing what is seemly, has covered and veiled what it has found hid in the frame of our body; like that door which Noah was bidden to make in the side of the ark; wherein we find a figure of the Church, and also of the human body, for through that door the remnants of food were cast out. Thus the Maker of our nature so thought of our modesty, and so guarded what was seemly and virtuous in our body, as to place what is unseemly behind, and to put it out of the sight of our eyes. Of this the Apostle says well: “Those members of the body which seem to be more feeble are necessary, and those members of the body which we think to be less honourable, upon these we bestow more abundant honour, and our uncomely parts have more abundant comeliness.” Truly, by following the guidance of nature, diligent care has added to the grace of the body. In another place I have gone more fully into this subject, and said that not only do we hide those parts which have been given us to hide, but also that we think it unseemly to mention by name their description, and the use of those members.
79. And if these parts are exposed to view by chance, modesty is violated; but if on purpose, it is reckoned as utter shamelessness. Wherefore Ham, Noah's son, brought disgrace upon himself; for he laughed when he saw his father naked, but they who covered their father received the gift of a blessing. For which cause, also, it was an ancient custom in Rome, and in many other states as well, that grown-up sons should not bathe with their parents, or sons-in-law with their fathers-in-law, in order that the great duty of reverence for parents should not be weakened. Many, however, cover themselves so far as they can in the baths, so that, where the whole body is bare, that part of it at least may be covered.
80. The priests, also, under the old law, as we read in Exodus, wore breeches, as it was told Moses by the Lord: “And you shall make them linen breeches to cover their shame: from the loins even to the thighs they shall reach, and Aaron and his sons shall wear them, when they enter into the tabernacle of witness, and when they come unto the altar of the holy place to offer sacrifice, that they lay not sin upon themselves and die.” Some of us are said still to observe this, but most explain it spiritually, and suppose it was said with a view to guarding modesty and preserving chastity.
Source: On the Duties of the Clergy (New Advent)