Did St. Augustine think sexual pleasure and concupiscence are identical? If not, why do some people seem to think this? Concupiscence is simply a disorder in which the body rebels against the rational soul; this is something completely different from pleasure.
No, St. Augustine doesn't say concupiscence is identical with venereal pleasure.
Although it is true "concupiscence is disorder in which the body rebels against the rational soul," one prominent manifestation of this disorder (which is not identical to the disorder) is seeking pleasure against the dictates of reason, which is the sin of lust.
pleasure in a good action is good, and in an evil action, evil; wherefore, as the marriage act is not evil in itself, neither will it be always a mortal sin to seek pleasure therein.
[operationis bonae est delectatio bona, et malae mala. Unde cum actus matrimonialis non sit per se malus; nec quaerere delectationem erit peccatum mortale semper.]
However, referencing St. Augustine, St. Thomas Aquinas seems not to distinguish concupiscence and venereal delectation (Summa Theologica II-II q. 153 a. 2 ad 2):
That venereal concupiscence and pleasure (concupiscentia et delectatio venereorum) are not subject to the command and moderation of reason, is due to the punishment of the first sin, inasmuch as the reason, for rebelling against God, deserved that its body should rebel against it, as Augustine says (De Civ. Dei xiii, 13).
To answer your great question. I think the common beliefs and understanding of the teaching of St. Augustine on the nature of original sin plays an important role why "sexual pleasure" is equated to "concupiscence'.
But you pointed out that concupiscence is simply a disorder in which the body rebels against the rational soul; this is something completely different from pleasure.
Your definition of concupiscence as simply a disorder not subject to reason or intellect is real and scriptures passages attested to it, as St.Paul stated this in Romans 7:15
"I do not understand what I do. For what I want to do, I do not do. But what I hate, I do."
From St.Paul confession we can see that pleasure or goodness can only be in state of good order when one wills the things he wants.This is true for Christians striving for holiness.
But how about those people, who does not strive for holiness but simply weak to control the concupiscence, is sexual pleasure possible by succombing to concupiscence? The answer is resoundingly YES! because they simply give in to it without resisting and desires it too, to satisfy sensual pleasures. In reality there's no incurred guilt or struggles in comparison to St.Paul.
If we go back to St. Augustine the concupiscence after the fall of man is defined as inclination to sin.
If one is by default or by fallen nature are inclined to desires sensual gratification outside of the control of the will then sexual pleasure can be equated to concupiscence.Because the body are inclined to it then concupiscence allows the bodily senses to satisfies it's pleasure. Eventhough the intellect sees it as a disorder not subject to will for those believer's striving for holiness.
As St. Augustine writes the following "
In a similar vein, Augustine notes that men did not have
in their members the law of their own concupiscence warring against the law of their mind'' (Ibid. II.37; Fathers of the Church). But what is concupiscence for Augustine? He writes that concupiscence isthe law of sin which remains in the members of this body of death'' and that this evil ``remains in our flesh, not by reason of the nature in which man was created by God and wisdom'' (Ibid. II.4; Fathers of the Church). Thus Augustine claims that concupiscence is precisely this disordering of members, and that it is certainly not a property of the man in the state of original justice.
In conclusion did St.Augustine think sexual pleasure and concupiscence are identical? If we based it on St.Augustine teaching on the nature of original sin and basing it on the behavior of the rational soul that does not strive for holiness sexual pleasure derived from concupiscence carries no guilt and conviction. Thus a rational soul enjoys fulfilling their sensual pleasures unless the grace of the Holy Spirit penetrates the soul to convict it. Then, satisfying sexual pleasures from concupiscence carries guilt and the disorder is exposed.God bless!