If we were just animals, no problem. If we were pure spirits, no problem. Concupiscence is a problem because we are spiritual animals. Neither angels nor beasts, we 'enjoy' dual residency in opposing spheres. The problem is not that the flesh is weak while the spirit is willing. The problem is that the spirit is fallen and wills the wrong thing: inordinate sensuous pleasure for its own sake.
The animal in us plays along supplying the playground for the spirit's perversity. The sins of the flesh do not originate there, but in the spirit. The flesh is merely the matter in which they are realized.
Or perhaps what I've just written, which is pretty standard MavPhil 'boilerplate,' is nonsense.
Maybe it is like this. Whatever 'spirituality' there is in us is merely a sickness that impedes our vitality and conjures up the ghosts of sin and guilt and free will and moral scrupulosity and talk of concupiscence. I never did get around to reading Ludwig Klages, Der Geist als Widersacher der Seele, though I can guess at his dark vision.
The powerful take what they want. The weak, who want what the powerful want but are too impotent to acquire it, invent morality.
As I said, a dark vision. And one to be found in the identity-political of the present day, both on the Right and on the Left.
You understand that I am not endorsing the dark vision.