I've always been puzzled about this. At an early age (already at 6-7) I was obsessed with both spanking and medical procedures involving the bottom. My parents spanked me frequently, and so it's not surprising that the experience stuck in my mind. But I don't ever remember having my temperature taken rectally, or getting a shot in the rear, or getting an enema. My doctor only had me take my shirt off for exams, and his nurse usually gave shots (even penicillin) in the leg.
I wonder: did the spankings create an situation where the combination of bending over, undressing, pain, and humiliation was erotically charged? Once that had happened, did that mean any situation with these qualities would be a source of anxiety and pleasure, even if I hadn't myself experienced it?
Or: I have heard that there are spankos who were not themselves spanked as children, but who got the fascination from witnessing it happening to others, or hearing accounts from their friends after the fact. I never myself got a shot in the bottom, but my mother often told me stories about when she was a child and penicillin was the standard treatment for many things. I can also remember one of my friends getting a shot and later dropping his pants to show me the results (an angry red dot with the round adhesive of the bandage around it). I think after hearing and seeing these things, my active imagination created a monster in my mind.
What do you think?