Do you have the context of that Aquinas quote? Do you know what he is trying to say in that quote?
Here is what the Catechism says about concupiscence. You can clearly see the Church teaches that it is not intself a sin, but it inclines us to sin.
405 Although it is proper to each individual,295 original sin does not have the character of a personal fault in any of Adam’s descendants. It is a deprivation of original holiness and justice, but human nature has not been totally corrupted: it is wounded in the natural powers proper to it, subject to ignorance, suffering and the dominion of death, and inclined to sin - an inclination to evil that is called concupiscence". Baptism, by imparting the life of Christ’s grace, erases original sin and turns a man back towards God, but the consequences for nature, weakened and inclined to evil, persist in man and summon him to spiritual battle.
978 "When we made our first profession of faith while receiving the holy Baptism that cleansed us, the forgiveness we received then was so full and complete that there remained in us absolutely nothing left to efface, neither original sin nor offenses committed by our own will, nor was there left any penalty to suffer in order to expiate them… Yet the grace of Baptism delivers no one from all the weakness of nature. On the contrary, we must still combat the movements of concupiscence that never cease leading us into evil "521
1263 By Baptism all sins are forgiven, original sin and all personal sins, as well as all punishment for sin.65 In those who have been reborn nothing remains that would impede their entry into the Kingdom of God, neither Adam’s sin, nor personal sin, nor the consequences of sin, the gravest of which is separation from God.
1264 Yet certain temporal consequences of sin remain in the baptized, such as suffering, illness, death, and such frailties inherent in life as weaknesses of character, and so on, as well as an inclination to sin that Tradition calls concupiscence, or metaphorically, “the tinder for sin” (fomes peccati); since concupiscence "is left for us to wrestle with, it cannot harm those who do not consent but manfully resist it by the grace of Jesus Christ."66 Indeed, "an athlete is not crowned unless he competes according to the rules."67
2515 Etymologically, “concupiscence” can refer to any intense form of human desire. Christian theology has given it a particular meaning: the movement of the sensitive appetite contrary to the operation of the human reason. the apostle St. Paul identifies it with the rebellion of the “flesh” against the "spirit."301 Concupiscence stems from the disobedience of the first sin. It unsettles man’s moral faculties and, without being in itself an offense, inclines man to commit sins.302
Here is the CCC alphabetical search that shows every sentence in the Catechism that shows the word “concupiscence”. I believe there are 16 refereces. Just click on the paragraph # to the left of the short line quotes.
vatican.va/archive/ENG0015/1/3S.HTM
