Did St. Augustine think sexual pleasure = concupiscence? 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.

Unlike concupiscence, which is an evil, coital pleasure in itself is morally neutral (Super Sent., lib. 4 d. 31 q. 2 a. 3 co. // suppl. q. 49 a. 6 co.):


  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).