Alleviating fears
With permission from reddit user u/Animator-Ill
I see the occasional post with people being worried about getting air in their urethra due to the inflation method. Hopefully I can alleviate this with an explanation of some of the physics involved.
Firstly, the urethra can seal against very high pressures. For instance, consider sinking to the bottom of a swimming pool. The pressure at this depth is far higher than the inside of an inflation device, but chlorinated water doesn't invade your urethra even at this pressure. A fully inflated party balloon, which is roughly equivalent to what we're doing to our skin, only equals the pressure experienced at a few inches of depth in the water.
Another way to think of it is, consider your urethra as the stem of a balloon. If you took a balloon and cut off the ring around the mouth of the opening so that the stem of the balloon could close completely flat, then tried blowing to inflate it, it would never happen. Without first peeling open the mouth of the stem (normally held open by the ring around the opening), you could blow as hard as you wanted, but the air pressure is only going to clamp that opening shut even harder. The balloon will shoot out of your mouth (or the inflation device off of your body) before any air could enter. The meatus is like the stem of a balloon. Unless its lips are blasted apart by an initial burst of air from an air pump and a high pressure stream of air shot directly into the urethra, the increased pressure of the inflated foreskin will only serve to close the opening more firmly.
This is why human beings are crushed when their pressurized protection measures fail extremely deep in the ocean, rather than exploding by having water invade the inside of their body. We're just very well designed to keep the outside environment of our body on the outside.