Wow... I was surprised I got so many comments about my stories about male arousals during physical exams. To answer a few of your questions, I've never heard of any issues or studies concerning males and repetitive motion injury (carpal tunnel) from masturbation. There are a number of documented cases with females however. (One even sued for Workmen's Comp in Florida). I would think the reason is simple. The nature of masturbation in men doesn't really involve a lot of joint movement (no pun intended). It really places more effort on the biceps than on the fingers.
Women however, tend to use their fingers extensively when arousing their clitorises, which can cause the injury. That, plus the fact that the average male orgasms much faster than the average female. Twenty minutes of rapid finger movement every day can have its toll.
Secondly, I must say I haven't ever become 'wet' or aroused during an exam of a particularly good looking man. That said, I can admit that I have used some of my patients as fantasies when I later masturbated, or had sex with my husband. (Sorry honey.. I think about George Clooney too!). Theo, would be happy to give you a thorough exam for testicular lumps, and make sure you are well versed in self examination. As for the rash, make an appointment with my office, and try not to scratch too much.
As far as other stories, I'll relate a couple. I had a teen a few years ago with a case of gynecomastia (this is where a boys nipples enlarge due to excessive hormone production during puberty) It can be very embarrassing for boys, as the nipples often bud out like a females, and tend to be very sensitive. He was very embarrassed of his nipples, and as I examined them to make sure that was the condition, I could tell that he had a very noticeable erection in his pants. He stammered out an apology, and I assured him it was no problem, that it as a natural reaction. 'This happened because I play with them so much when I jerk off right?' I again assured him that it was biological and not anything he did. He seemed relieved, then added 'So it's okay if I still play with them right?' I got a good laugh later thinking about this boy masturbating while tugging away on his rather ample nipples. Another common thing I see in teen boys in exams are raw marks where they have literally rubbed themselves raw while masturbating. I need to check the injury to ensure it isn't STD related, and ask if they hurt themselves masturbating. I tell them that it is natural to masturbate, but they need to be careful of the sensitive skin, and give them a sample tube of lube jelly. the looks I get are priceless!
I have a sports team physical I'm doing Thursday at a local high School, and if there are any interesting tales, I'll come back and relate!