Objection 1. It would seem that purity does not belong especially to chastity. For Augustine says (De Civ. Dei i, 18) that "purity is a virtue of the soul." Therefore it is not something belonging to chastity, but is of itself a virtue distinct from chastity.
Objection 2. Further, "pudicitia" [purity] is derived from "pudor," which is equivalent to shame. Now shame, according to Damascene [De Fide Orth. ii, 15, is about a disgraceful act, and this is common to all sinful acts. Therefore purity belongs no more to chastity than to the other virtues.
Objection 3. Further, the Philosopher says (Ethic. iii, 12) that "every kind of intemperance is most deserving of reproach." Now it would seem to belong to purity to avoid all that is deserving of reproach. Therefore purity belongs to all the parts of temperance, and not especially to chastity.
On the contrary, Augustine says (De Perseverantia xx): "We must give praise to purity, that he who has ears to hear, may put to none but a lawful use the organs intended for procreation." Now the use of these organs is the proper matter of chastity. Therefore purity belongs properly to chastity.
I answer that, As stated above (Objection 2), "pudicitia" [purity] takes its name from "pudor," which signifies shame. Hence purity must needs be properly about the things of which man is most ashamed. Now men are most ashamed of venereal acts, as Augustine remarks (De Civ. Dei xiv, 18), so much so that even the conjugal act, which is adorned by the honesty [Cf. 145] of marriage, is not devoid of shame: and this because the movement of the organs of generation is not subject to the command of reason, as are the movements of the other external members. Now man is ashamed not only of this sexual union but also of all the signs thereof, as the Philosopher observes (Rhet. ii, 6). Consequently purity regards venereal matters properly, and especially the signs thereof, such as impure looks, kisses, and touches. And since the latter are more wont to be observed, purity regards rather these external signs, while chastity regards rather sexual union. Therefore purity is directed to chastity, not as a virtue distinct therefrom, but as expressing a circumstance of chastity. Nevertheless the one is sometimes used to designate the other.
Reply to Objection 1. Augustine is here speaking of purity as designating chastity.
Reply to Objection 2. Although every vice has a certain disgrace, the vices of intemperance are especially disgraceful, as stated above (Question 142, Article 4).
Reply to Objection 3. Among the vices of intemperance, venereal sins are most deserving of reproach, both on account of the insubordination of the genital organs, and because by these sins especially, the reason is absorbed.
Source: Summa Theologica (New Advent)