i don't think it's possible to accurately quantify this rate (%) for several reasons

- men tend not to openly talk about (admit to) catching STDs
- one man's mild-STD is another man's nuissance (is a skin rash worth reporting?)
- ultimately what you're asking is what are the odds of ONE hobbyist catching STDs over a stretch of time and since most VD clinics keep anonymous records it's hard to determine the frequency or recurrence in one hobbyist

if you follow all the recommended precautions (including restricting your providers to the "right" kind) you shouldn't have much to worry about. in 15+ years of hobbying (including some reckless indiscretions in my youth with the "wrong" kind of providers) i managed to emerge unscathed (worst case worry i had was a mild skin rash caused by allergic reactions and/or too much friction -- although it sure FELT like crabs at the time!)

Since none of us wants to be hurt or killed hobbying we tend to obey track rules and always wear our helmets.

 Personally I feel safer with professionals than amateurs .

So far the worst I have sustained was crabs, and that was from a girlfriend?! I am selective, do not do BBBJ or DATY and cover up for all recreational activities. Immediately after the main event I use antiseptic wipes, Purell antiseptic hand cleaner and pee on the way to the shower. I think statistics are impossible to come by. Besides, the stat i care about is my own track record. Play safe and be brave at all times.