Yeah, but walking across the street, you can see the bus coming. The risks are quite visible, and the repurcussions immediate. You don't walk across the street, and wake up three days later with an unusual burning sensation...
That being said, it is best to quantify your risks. Nothing is 100% safe, even with a long term monogamous partner. Should you go down on a street walker just afdter you've had a dental cleaning? Not too safe. Should you orally pleasure a lady who keeps an upscale incall, is always showered and fresh when you arrive, and charges a normal to above normal rate? Risks are a lot lower, IMHO. Are the risks in the second scenario low enough for you? That's your decision.
No SP will ever be 100% safe in any activity. Things happen to expose those risks. It is good to know what the risks are, and threads like these help increase that knowledge. Its just a case of deciding what risks to take, and living with the consequences.
Rubin