Sorry for the length, but I jack off to this experience all the time.
Here is my expeerience.
I had to get a complete physical about a year ago for the Laborers' union insurance. I went to a large clinic. They do plenty of physicals each day. When it was my turn, a pleasant looking athletic young Doctor called my name. He was 25 he told me later on.
We went through the process and when it came for a prostate check, he had me stand at the foot of the examining table. He then greased up his gloved finger and greased up my butt hole taking his time, all the while talking to me as how this will not hurt and it may even feel good.
I was naked and he told me that he admired my body. For a 42-year-old guy, I was in good shape. The Doc proceeded to insert his finger slowly. He then started to massage my prostate taking his time. It didn't hurt at all. In fact, I got a semi-hardon and when he continued to slowly massage my prostate, I got the hardest, thickest hardon in years. The Doc saw my hardon and after admiring it, he said it was common for guys to get hardons.
He continued the massage and I started to drip pre-com. He evidentlly saw this also, and he bore down slightly harder and I shot my wad. I spurt 3 long jets of cum on the paper towel covering of the exam table. I arched my back turned around and apologized to him. He again said this was natural and common.
While looking at him I noticed a large bulge in his pants. He had a hardon, too! I smiled at him and he grinned back. I reached down and fondled his cock and balls and he took his cock outta of his pants and I jacked him off and he spurt HUGE jets of cum over my cum. The paper covering was saturated with our cum. I told him I was returning the favor. We both laughed and I got dresssed. Needless to say, I passed the exam.
(P.S. He told me that he hoped that would happen. He also said that happens to him at least a couple of times a week!)