I've never seen a dental dam, either. I have heard of using a sheet of Saran Wrap to cover the vagina. I think if I were that worried with a lady, I'd just suddenly remember an appointment I had to take my aunt to the emergency room.
Seriously, I asked a doctor this question.
He said there is little liklihood of transmitting an STD orally because the body has strong defenses in the form of stomach acid and other pretty powerful chemicals in the mouth. That's why mamma lions lick their cubs' wounds, to prevent infection.
The AIDS virus, he told me, dies very quickly in such an environment. I don't remember if he told me the risks of getting other STDs or not, but you might do some research on the net.
We have always restricted our exposure by partner selection and playing only with couples. Swingers, as a group, are freer from STDs than the general population, I think. There is an unscientific STD survey on this board. As I recall, there were very few reports of getting an STD from swingers.
Mr. Alura