For in the soul alone are beauty and deformity shown. Wherefore also only the virtuous man is really beautiful and good. And it is laid down as a dogma, that only the beautiful is good. And excellence alone appears through the beautiful body, and blossoms out in the flesh, exhibiting the amiable comeliness of self-control, whenever the character like a beam of light gleams in the form. For the beauty of each plant and animal consists in its individual excellence. And the excellence of man is righteousness, and temperance, and manliness, and godliness.
All the cardinal virtues (mentioned in the last sentence) are beautiful; but notice that it is the 'amiable comeliness of self-control' that expresses this beauty of character in an outward way.
Aquinas himself famously identifies three conditions of beauty: integrity or completeness, proportion or harmony, and brightness or clarity. A little thought shows that each of these is operative in the virtue of temperance.
(1) Proportion is the most obvious condition. While every virtue can be considered a kind of moderation and appropriateness, as we see in Aristotle's account of virtue as expressing a mean, temperance is the virtue that is most obviously concerned with moderation and finding the appropriate balance among a variety of factors; the very name of the virtue highlights this moderating and balancing. To act temperately is to act in proportion to what is needful in human life; temperance is the proportioning virtue. Thus Ambrose says (De officiis 1.45):
Let us then hold fast modesty, and that moderation which adds to the beauty of the whole of life. For it is no light thing in every matter to preserve due measure and to bring about order, wherein that is plainly conspicuous which we call decorum, or what is seemly.
(2) When Augustine talks about temperance in Of the Morals of the Catholic Church, he emphasizes the aspect of temperance that suggests integrity: "temperance is love giving itself entirely to that which is loved" and "temperance is love keeping itself entire and incorrupt for God" are his two descriptions (Chapter 15) of the Christian version of the virtue. Temperance goes with wholeheartedness, since it is a virtue in which we are especially concerned with eliminating even the appearance of inconsistency with good character.
A further sign that temperance especially exhibits the condition of integrity is its association with tranquillity of mind. This is an old and constant association; Cicero in De Finibus XIV tells us that temperance "brings peace to the mind, and soothes and tranquillizes them by what I may call a kind of concord"; he takes this concord, in fact, to be precisely that at which temperance aims, and for the sake of which it is to be sought. But this sort of tranquillity is a kind of resting in the good, or a completion of good action. Temperance is thus concerned with bringing our character to a kind of completeness or wholeness. And we see the same thing from the reverse direction. When we speak of moral integrity, we are often talking about the kind of inner self-sufficiency by which one can maintain principles even in the face of great temptation.
(3) It is clear as well that temperance is concerned with manifestation or communication. As Ambrose says (De officiis 1.47), "If any one preserves an even tenor in the whole of life, and method in all that he does, and sees there is order and consistency in his words and moderation in his deeds, then what is seemly stands forth conspicuous in his life and shines forth as in some mirror." Being virtuous is certainly more important than appearing virtuous, but the virtue of temperance does concern itself with the question of how virtue appears. Since we are social creatures, one of the needs of human life is to communicate well, and we therefore reasonably restrain ourselves from things that, however they might be in themselves, could give others the wrong idea about right or wrong, good or bad. We see this very easily in how adults deal with children -- they will restrain themselves in matters that are not in themselves wrong simply because children are not yet ready to understand them. We see it in a different way in friendships, in which friends will deliberately avoid things that they normally enjoy, out courtesy to their friends. And we also see it in the way people worry about what they are condoning, or about the message they are sending with their actions.
Given that temperance is concerned with the beautiful, it will often not involve anything obligatory. To be sure, there are things that are simply inconsistent with temperance, and likewise things that are simply necessary for it, but much of the restraint of temperance will concern things not themselves necessarily bad. To use Jane Austen's phrase again, temperance often involves 'the restraint of sentiments which are not in themselves illaudable'. Unlike justice, which concerns itself with obligation almost everywhere, temperance involves a sort of artistic appreciation and touch, being concerned not just with doing what one should, but doing it in a way appropriate to itself. Indeed, it would perhaps be reasonable to say that good taste is to temperance as law or obligation is to justice.