I think for me it was when I was around 15 years old. I needed to get a sports physical, they allowed either the group physical or approval from your GP. I didn't want to go to what would undoubtedly be a sausagefest so I went to my GP.
They had me waiting in the exam room for a while just in my underwear. Finally a nurse came in, who happened to be a friend of the family, and I always considered her to be kind of a milf. I always thought she looked like Morticia from the Addams Family. She said she had some paperwork for me, it was about it being for sports, and told me she was going to run some basic tests & such on me.
She checked weight, height, temperature, blood pressure, the basics really, but she did another test which I can only assume was range of motion/flexibility. She had me touch my toes, and move in various positions, then she had me get on the table and do one where I pulled my knee as close to my chest as possible and hold it. I was wearing boxers, they were kind of old and getting small on me, so when I pulled up my knee it all stretched tight and I popped out of the leg hole in all my glory.
She saw it pop out and said some like "Oh dear, your penis is hanging out". And I don't know what she was thinking but she just grabbed my penis and balls and tried to put em back in, it didn't work. So she just went to move on and had me do my other leg, and it all popped right back out again. At this point natural reactions started happening and was clearly starting to get an erection. She had me stand back up for the last couple tests which I did at full mast, the rest of the time I kept seeing her eyes wander down to my penis.
Ever since then any time I see or think of a nurse or doctor all I can remember is the exposure which I rather enjoyed and her touching me and unsuccessfully trying to cover me back up. Enjoyed medical situations ever since.