Do practices of mortification represent a gnostic contempt for the body?
Don't ascetical practices such as fasting represent a sort of Gnostic contempt for the body? I know Paul talks about struggling against "the flesh," but doesn't this refer to the sin nature, not the body?
No. That is the quasi-Gnostic idea—the notion of a disembodied "sin nature" and the banishment of the moral struggle from the physical realm. We have no "sin nature," if by that is meant a principle in us that is positively sinful in and of itself. Rather, we have human nature, composed of body and spirit, good in and of itself but wounded by the effects of sin.
This wound disorders our natural appetites or desires, weakens our wills, and darkens our minds. We are attracted too strongly to good things and are attracted to things that are not good for us. We may crave too much food, or we may crave bad foods—sometimes even things that are not food at all. Such disordered attraction may be a source of temptation, an occasion of sin, but it is not sinful in itself.
Yet in order to fight against sin, we must avoid occasions of sin as well as actual sin. Concupiscence (disordered desires) is not sinful in itself, but we still must fight against it. We must struggle to strengthen our weakened will and master our appetites so we will be able to resist temptation. Acts of self-denial or asceticism, done in grace, help us do this. Thus Jesus instructs us on how to fast (Mt 6:16–18). We are expected to engage in ascetical practices of one sort or another.