The risk is great for both parties when not covered just, like bbbj!

GYT

The only time I used a dental dam is the closest I've ever been of understanding why condom can get in the way of pleasure...and the first time I could relate with the gentleman who has to finish manually...not my idea of best stimuli...also when I was on the giving end (it was a threesome) the flavored dam that was used might not have been a good idea, food and sex do not mix for me...

...for both the giver and the receiver.  

Check out the CDC's stats on it.