Just sitting around amusing myself by thinking of hideous tortures.
Here's one that comes to mind for a man's penis:
Picture a hollow tube that will be inserted into the urethra. It has a rounded end to enable it to slide in easily. The surface, though, is covered with tiny spikes. The spikes are sharp, and sharply angled backward. So this tube slides easily in, but can never be removed without totally shredding the penis from the inside.
The victim will have to wear this for the rest of his life, however short or long that may be (unless, of course, the penis is removed altogether—a merciful act in the circumstances, for which he will undoubtedly beg). The hollow tube allows urination. Any movement of the penis or touch upon it causes excruciating pain from the spikes. That means walking or any other movement of the body that causes the penis to sway from side to side is agonizing.
As the outer surface of the penis is unaffected by this, other tortures may of course be applied there.
Your imagination can take it from there!