I've read a few posts on this idea on this site and at voy forums. I can't speak for any other guy, I can only speak for myself. I've been seen and/or examined by a lot of female caregivers. Usually, I don't get an erection. Maybe this is because it's not new to me. But, there have been times when I had a full-scale erection during an exam. The reactions I've gotten have been:
(1) My erection was ignored as if it wasn't happening
(2) I was told, "Don't worry about it, it happens sometimes."
(3) Freak out. This has happened twice. Once when I was getting prepped for a cystoscopy, the nurse doing the prep left the room and "told on me" to the doctor. The second time, the doctor started screaming at me and left the room for 45 minutes so I could "calm down."
I read accounts where the doctor smiles or gets playful or derives obvious enjoyment from the moment, but I've never gotten those reactions.