Objection 1. It would seem that sins do not differ in species, according to their objects. For acts are said to be good or evil, in relation, chiefly, to their end, as shown above (1, 3; 18, A4,6). Since then sin is nothing else than a bad human act, as stated above (Question 71, Article 1), it seems that sins should differ specifically according to their ends rather than according to their objects.
Objection 2. Further, evil, being a privation, differs specifically according to the different species of opposites. Now sin is an evil in the genus of human acts. Therefore sins differ specifically according to their opposites rather than according to their objects.
Objection 3. Further, if sins differed specifically according to their objects, it would be impossible to find the same specific sin with diverse objects: and yet such sins are to be found. For pride is about things spiritual and material as Gregory says (Moral. xxxiv, 18); and avarice is about different kinds of things. Therefore sins do not differ in species according to their objects.
On the contrary, "Sin is a word, deed, or desire against God's law." Now words, deeds, and desires differ in species according to their various objects: since acts differ by their objects, as stated above (Question 18, Article 2). Therefore sins, also differ in species according to their objects.
I answer that, As stated above (Question 71, Article 6), two things concur in the nature of sin, viz. the voluntary act, and its inordinateness, which consists in departing from God's law. Of these two, one is referred essentially to the sinner, who intends such and such an act in such and such matter; while the other, viz. the inordinateness of the act, is referred accidentally to the intention of the sinner, for "no one acts intending evil," as Dionysius declares (Div. Nom. iv). Now it is evident that a thing derives its species from that which is essential and not from that which is accidental: because what is accidental is outside the specific nature. Consequently sins differ specifically on the part of the voluntary acts rather than of the inordinateness inherent to sin. Now voluntary acts differ in species according to their objects, as was proved above (Question 18, Article 2). Therefore it follows that sins are properly distinguished in species by their objects.
Reply to Objection 1. The aspect of good is found chiefly in the end: and therefore the end stands in the relation of object to the act of the will which is at the root of every sin. Consequently it amounts to the same whether sins differ by their objects or by their ends.
Reply to Objection 2. Sin is not a pure privation but an act deprived of its due order: hence sins differ specifically according to their objects of their acts rather than according to their opposites, although, even if they were distinguished in reference to their opposite virtues, it would come to the same: since virtues differ specifically according to their objects, as stated above (Question 60, Article 5).
Reply to Objection 3. In various things, differing in species or genus, nothing hinders our finding one formal aspect of the object, from which aspect sin receives its species. It is thus that pride seeks excellence in reference to various things; and avarice seeks abundance of things adapted to human use.
Source: Summa Theologica (New Advent)