A twofold division of what is seemly is given. Next it is shown that what is according to nature is virtuous, and what is otherwise must be looked on as shameful. This division is explained by examples.
231. Seemliness, therefore, which stands conspicuous has a twofold division. For there is what we may call a general seemliness, which is diffused through all that is virtuous, and is seen, as one may say, in the whole body. It is also individual, and shows itself clearly in some particular part. The first has a consistent form and the perfection of what is virtuous harmonizing in every action. For all its life is consistent with itself, and there is no discrepancy in anything. The other is concerned when there is any special action done in a virtuous course of life.
232. At the same time let us note that it is seemly to live in accordance with nature, and to pass our time in accordance with it, and that whatever is contrary to nature is shameful. For the Apostle asks: “Is it comely that a woman pray unto God uncovered; does not nature itself teach you that if a man have long hair, it is a shame unto him? For it is contrary to nature.” And again he says: “If a woman have long hair, it is a glory unto her.” It is according to nature, since her hair is given her for a veil, for it is a natural veil. Thus nature arranges for us both character and appearance, and we ought to observe her directions. Would that we could guard her innocence, and not change what we have received by our wickedness!
233. We have that general seemliness; for God made the beauty of this world. We have it also in its parts; for when God made the light, and marked off the day from the night, when He made heaven, and separated land and seas, when He set the sun and moon and stars to shine on the earth, He approved of them all one by one. Therefore this comeliness, which shone forth in each single part of the world, was resplendent in the whole, as the Book of Wisdom shows, saying: “I existed, in whom He rejoiced when He was glad at the completion of the world.” Likewise also in the building up of the human body each single member is pleasing, but the right adjustment of the members all together delights us far more. For thus they seem to be united and fitted in one harmonious whole.
Source: On the Duties of the Clergy (New Advent)