Objection 1. It would seem that even virtuous men can be ashamed. For contraries have contrary effects. Now those who excel in wickedness are not ashamed, according to Jeremiah 3:3, "Thou hadst a harlot's forehead, thou wouldst not blush." Therefore those who are virtuous are more inclined to be ashamed.
Objection 2. Further, the Philosopher says (Rhet. ii, 6) that "men are ashamed not only of vice, but also of the signs of evil": and this happens also in the virtuous. Therefore virtuous men can be ashamed.
Objection 3. Further, shamefacedness is "fear of disgrace" [Ethic. iv, 9. Now virtuous people may happen to be ignominious, for instance if they are slandered, or if they suffer reproach undeservedly. Therefore a virtuous man can be ashamed.
Objection 4. Further, shamefacedness is a part of temperance, as stated above (143). Now a part is not separated from its whole. Since then temperance is in a virtuous man, it means that shamefacedness is also.
On the contrary, The Philosopher says (Ethic. iv, 9) that a "virtuous man is not shamefaced."
I answer that, As stated above (1 and 2) shamefacedness is fear of some disgrace. Now it may happen in two ways that an evil is not feared: first, because it is not reckoned an evil; secondly because one reckons it impossible with regard to oneself, or as not difficult to avoid.
Accordingly shame may be lacking in a person in two ways. First, because the things that should make him ashamed are not deemed by him to be disgraceful; and in this way those who are steeped in sin are without shame, for instead of disapproving of their sins, they boast of them. Secondly, because they apprehend disgrace as impossible to themselves, or as easy to avoid. On this way the old and the virtuous are not shamefaced. Yet they are so disposed, that if there were anything disgraceful in them they would be ashamed of it. Wherefore the Philosopher says (Ethic. iv, 9) that "shame is in the virtuous hypothetically."
Reply to Objection 1. Lack of shame occurs in the best and in the worst men through different causes, as stated in the Article. On the average men it is found, in so far as they have a certain love of good, and yet are not altogether free from evil.
Reply to Objection 2. It belongs to the virtuous man to avoid not only vice, but also whatever has the semblance of vice, according to 1 Thessalonians 5:22, "From all appearance of evil refrain yourselves." The Philosopher, too, says (Ethic. iv, 9) that the virtuous man should avoid "not only what is really evil, but also those things that are regarded as evil."
Reply to Objection 3. As stated above (1, ad 1) the virtuous man despises ignominy and reproach, as being things he does not deserve, wherefore he is not much ashamed of them. Nevertheless, to a certain extent, shame, like the other passions, may forestall reason.
Reply to Objection 4. Shamefacedness is a part of temperance, not as though it entered into its essence, but as a disposition to it: wherefore Ambrose says (De Offic. i, 43) that "shamefacedness lays the first foundation of temperance," by inspiring man with the horror of whatever is disgraceful.
Source: Summa Theologica (New Advent)