I've got some news for you, Tony:
Sex is a two-way street. If you are not at least as concerned about what *you* might transmit to a partner, or your *partner's* chances of getting something from you, keep your pants zipped. Seriously.
That said, your liability for catching an STD from a partner when you are *receiving* unprotected oral sex depends entirely on what kinds of STDs your partner might have been exposed to. If your partner had chlamydia in the throat (fairly common, and people usually never know they have it), or herpes (cold sores) on the mouth or lips, your chances of getting an STD are pretty damned good, actually.
Thing is... chlamydia is called 'the silent STD' for a reason. Most folks who get it never know it until it's caused enough scarring and damage that they have other significant health problems. And herpes isn't always visually identifiable, either, even though we usually think of oral herpes as cold sores. The skin where a lesion is about to occur, with oral herpes, can start shedding active viruses well before a lesion actually develops, so it's very possible to get herpes from someone who doesn't look like they have it.
What's the moral of the story? Simply put, that YOU HAVE NO WAY OF KNOWING WHETHER A PARTNER HAS AN STD unless you have been monogamously partnered with that person and you have both been tested clear for all STDs at least twice, at six-month intervals.
That's the long and short of it.
And since you don't know whether a given partner may or may not have an STD, you simply have no idea whether or not the next mouth you want to put your penis in is going to be the one that's going to be infected with something you'd rather not catch. I can assure you that your partner would also rather not catch whatever *you* might have been exposed to, too, so why not protect both of you?
Wear a condom, already. It's a no-brainer.
Start a Revolution -- Stop Hating Your Body!