While I think denuseri gave some excellent advice this one came as a surprise:
Quote Originally Posted by denuseri View Post

Applying the mouth to his cock ... can inadvertently introduce the bacterium Escherichia coli ("E. coli") into the urethra ...
Huh? Ass <--> Mouth <--> Urethra? Do I miss something here?

Also, all in all I'd be more worried about ass to pussy. And, finally, one of the simplest solutions is to use a condom, then pull it off before it goes in the mouth.