When my father died we let them have his eyes for medical practice. How else is a medical student going to learn properly? It would be just as foolish for me to not let the medical students examine my gallbladder after it was taken out, just because it was a part of my body. They aren't going to desecrate it. Let's not get too scrupulous. In fact I thought it was a good thing that may help a future patient. If I donated my body to medicine (although I don't know why they would want this broken down husk), I wouldn't mind. My family doctor would make sure it would not be used for nefarious purposes, only education.
Or would you like new doctors practice on you (with living organs) instead? I would like my doctor to have at least some familiarity with an organ before he started in on mine.
Oh and that rules out a body never having an autopsy either because they often examine the body, cutting and so on, sometimes with intense examinations, and organ removal, occasionally with new doctors observing and learning as well. As long as the body is disposed of properly -- family requests and burial, there shouldn't be a problem.