The primary difference between Catholic and Protestant theology on the issue of concupiscence is that Protestants consider concupiscence to be sinful, whereas Catholics believe it to be highly likely to cause sin, though not sinful in itself.
This difference is intimately tied with the different traditions on original sin. Protestantism holds that the original prelapsarian nature of humanity was an innate tendency to good; the special relationship Adam and Eve enjoyed with God was due not to some supernatural gift, but to their own natures. Hence, in the Protestant view, the Fall was not the destruction of a supernatural gift, leaving humanity's nature to work unimpeded, but rather the corruption of that nature itself. Since the present nature of humans is corrupted from their original nature, it follows that it is not good, but rather evil (although some good may still remain). Thus, in the Protestant view, concupiscence is evil in itself.
Catholicism, by contrast, teaches that humanity's original nature is good (CCC 374). This condition is referred to as original righteousness. After the Fall this gift was lost, (see original sin) but in the Catholic view, human nature cannot be called evil, because it still remains a natural creation of God. Despite the fact that sin usually results, Catholic theology teaches that human nature itself is not the cause of sin, although once it comes into contact with sin it may produce more sin, just as a flammable substance may be easily ignited by a fire.
The difference in views also extends to the relationship between concupiscence and original sin. In the Protestant view, original sin is concupiscence inherited from Adam and Eve. It is never fully eliminated in this life, although sanctifying grace helps to eliminate it gradually. Since concupiscence is not evil in the Catholic view, it cannot be original sin. Rather, original sin is the real and actual sin of Adam, passed on to his descendants; rather than remaining until death (or in the case of the damned, for all eternity), it can be removed by the sacrament of baptism. (For more information, see original sin.)
Another reason for the differing views of Protestants and Catholics on concupiscence is their position on sin in general. Protestants (or at least the magisterial reformers; some modern-day Protestants would not accept this position) hold that one can be guilty of sin even if it is not voluntary; Catholics, by contrast, traditionally believe that one is subjectively guilty of sin only when the sin is voluntary. The Scholastics and magisterial reformers have different views on the issue of what is voluntary and what is not: the Catholic Scholastics considered the emotions of love, hate, like and dislike to be acts of will or choice, while the Protestant reformers did not. The Bible specifies that attitudes as well as actions may be sinful. By the Catholic position that one's attitudes are acts of will, sinful attitudes are voluntary. By the magisterial reformer view that these attitudes are involuntary, some sins are involuntary as well. Since man's nature (and therefore concupiscence) is not voluntarily chosen, Catholics do not consider it to be sinful; the reformers believe that, since some sins are involuntary, it can be.
Protestants believe that concupiscence is sinful, indeed, they believe it to be the primary type of sin; thus they most often refer to it simply as sin, or, to distinguish it from particular sinful acts, as "man's sinful nature". Thus, concupiscence as a distinct term is more likely to be used by Catholics.