It does happen. I, myself have fallen into the trap of skin-skin sensation with providers. Nothing I can tell you comes close to justifying my idiotic actions. What I can say, from experience, is that after all is said and done, the momentary pleasure is NOT worth the constant worrying and anticipation of possibly contracting a STD.
At one point during my mongering career, I was taking a Rx medication that hindered my sexual feeling in my groin area. I could still get "it" up, but while using a condom, I had little or no sensation at all. Therefore, I would either settle for a HJ, a BBBJ, and in some instances BBFS. It was stupid, YES!, but at the time, it was the only way I could get off.
My sensations have sense come back and the enticement of BBFS is still there. However, you have to discipline yourself going in and stick to your thought patterns of having safe sex.
I know there are many mongers that claim they've never used rubbers and are just fine. I've taken my chances, and fortunately, I'm still alive and healthy today. I don't want to become part of the growing number of statistics though, so nowadays, I strap up at all times. I recommend you all do too. Unfortunately, some will learn the hard way.
Good Luck and be safe!
-Bmalibu