"It's honestly not the a-grown-man-is-touching-my-balls part that upsets me, really. (Although considering how many old men have hit on me, it really should.) It's not even the erection issue*. It's that he has to leave so I can change into the robe--which I think is stupid anyway, considering that he's about to be staring directly at my crotch as he touches it. And when he leaves, he goes and checks on other patients, which makes my time spent waiting at the doctor's office even longer.
On the other hand, at least medical science has progressed past the point of leeches. No telling how much time I'd be waiting if people were getting leeches applied to them.
*My crotch has issues. If other people--man or woman, it doesn't give a shit--are in viewing distance of my naked penis, I get hard. This is either my exhibitionism at work, or my penis just assumes that if it's unclothed and there's another person in the room, we're going to have sex, so it might as well get ready."