and if you are barebacking (and have a brain) you trust this person implicitly. Makes a big difference.
Unfortunately the two don't always go together. I know many men who lose the ability to think when they get an erection. Not enough blood for both I guess.
If you know each others history and are comfortable with the decision, then do whatever you want. If you don't know, or have doubts about their honesty, protect yourself. Ignorance is no excuse for pregnancy or sexually transmitted diseases.