Why would anybody put something in their urethra? Wouldn’t that hurt? Would it be mainly masochists who would do this?
Some people must get gratification from it, although it seems to be a lot less common than the practice of putting things up the bum. Perhaps it's a mental illness.
I knew a gynecologist who had to operate on a woman's bladder to remove a leek that she had stuck all the way up the urethra. It had been in the bladder for so long, it had calcified and basically turned to stone.