I think it's to do with their familiarity/appreciation of the human body.
Of course it could also be to do with the amount of guys that get a boner when receiving a nice relaxing bedbath from a lovely young nurse
I smashed my hip up many years ago. I was on a morphine epidural for the pain, and the nurse was very impressed with my physical fortitude when I rose to the occasion as she delicately dabbed around my groin. I was failry young and went bright red in the face, these days I'd just look up at her and wink smilingly