If they are jerking it with a finger in my ass, I have no problem staying hard and shooting a good load, I want to have a hands free cum while my prostate gets rubbed

I had a prostate exam at my doctors office. He told me I might get an erection and it was fine if I did. He felt around in there and I got rock hard. When he was done I was at full erection and leaking. I was so horny with him looking at my erection. I think he could tell that I wanted to do something with it. He handed me some tissue and he moved in front of the door so nobody would open the door. He turned his back and I literally just touched my erection a couple of times and had a really big orgasm into the tissue. I just went ahead and grunted as the cum shot into the tissue. He asked me if I was better now. I said yes. He assured me again that the erection and orgasm were completely normal. He said some men actually have one during the exam and that is ok too.