I wish there was more balanced information on this topic. Most if not all websites (government or otherwise) indicate what the black or white, all or nothing risks, are. But the fact of the matter is, for every disease, there are different rates of transmission, depending on whether you're the giver, or the receiver, and whether the infected person is more symptomatic or not (e.g. cold sores).
Unfortunately, these important nuances are rarely disclosed or discussed b/c I believe the ultimate goal of these websites is to promote covered sex at all times.
I was shocked to find out that, for example, that the chances of a woman transmitting HIV to a man through unprotected vaginal intercourse is about 1 in 2,500:
HIV Risk Behaviors | HIV Risk and Prevention Estimates | HIV Risk and Prevention | HIV/AIDS | CDC. Mind blowing when you think about that...you would assume that it was damn near 100% every time.
There must be similar stats out there for receiving oral, but, difficult to find.