The first surgery I had for kidney stones, they went in through the urethra. They must not have used enough anesthesia to knock me out enough, because I awoke to see two people holding my cock and shoving what looked like a garden hose in it. I remember saying "OOOWWWWWW", then a gloved hand pushed my head back. Then I woke a couple hours later and vomited up my spleen.
If you think you want to shove something in there for fun, let me tell you it isn't.