MD here ......
Firstly, you will not piss. ...
DONT PANIC..... YOU WON'T PISS YOURSELF !!!!!!
That is not always true, as I've had several bottoms loose control and unintentionally urinate while being fucked. While it happens rarely during fucking, it's quite common during fisting, particularly if the bottom is a novice and doesn't empty his bladder first. Note that we are talking about much deeper penetration than during a prostate exam.
It is true that stimulation of the prostate does create a sensation like the need to urinate in many men. Similarly, if your protate is inflamed or irritated, you will often have constant a sensation of needing to urinate, when the bladder is actually empty. However, rough and deep fucking, or fisting, can result in an actual release of urine, particularly if the bladder is quite full. I believe this is because the penis (or fist, as the case may be) does actually press on the bladder, or at least the neck of the bladder above the prostate.
I have one rather humerous story about this. I was fucking a friend of mine at a sex club. We had gone to the sex club because they had rooms with a sling, and neither of us had a sling at home. The intention had been to fist him later, but while I was fucking him, he lost control and pissed all over himself. He was embarassed enough about that. However, as we started to clean up, he noticed a puddle on the floor under the sling, and then found the puddle was very large. Finally, he discovered that his pants, which had fallen to the floor, were soaked.
He was aware that he had pissed suring sex, which explained why he and the sling were wet. But he was horrified to see the volume of the puddle on the floor, and began to get very embarassed that he could have pissed so much!
As it turned out, he was personally resposible for only a small portion of the big puddle. Apparently someone had flooded the restroom across the hall, and some of the water had come into the sling room under the door. He was much relieved to realize that he had wet just himself, not the whole room!
So, the best plan is to make sure that your bladder is not too full before sex. Even as a total top, I find that good advice for myself. There has been more than one time when my bottom decided to fall asleep on top of daddy after sex, and I began to get very uncomfortable from his weight pressing on my bladder.