About 6 years ago, I had an emergency medical problem and had to be catheterized in the hospital. I had no sensation that my bladder had almost a liter and a half in it, but my symptoms went away very quickly as my bladder drained. As far as discomfort, it really didn't bother me at all. They used a 16FR catheter (about 5.3 mm in diameter) and it went in very easily. A number of years earlier, I had to be cathed in the hospital and a female doctor that probably was a member of the Gestapo jammed it in me so fast, without pausing as it hit my prostate and sphincter, that the pain was 11 on a scale of 10. If I was not so sick, I would have slugged her. In any case, the nurse that put it in me this time went very slow and used a lot of lube, she paused often, asked me how I was doing, and proceeded this way until it hit the neck of the bladder. At this point, she just pushed very gently until I felt the muscles relax and the catheter just entered the bladder with no trauma or pain. In fact, it kinda felt pretty good.
I had to come home with the catheter and had it in for 4 weeks until it was decided I still needed the catheter and it was time for a change. Things were getting a little raw where the catheter went into me and along my urethra, so imagine my surprise when they said that moving to a larger size would be more comfortable! The explanation was that there would be less movement between me and the catheter. They used a 22FR catheter (approximately 7.3 mm in diameter) and it was interesting getting that in. The biggest hurdle was getting it to first go into the penis, but once it was in the first quarter inch, it went very smoothly. The friction of this big catheter going through me started to cause me to get hard. Great, not only was this embarrassing for me with a pretty cute nurse holding me tight, but the cath now had to traverse a much greater distance! Hitting the bladder was also interesting. I was trying keep a hard-on under control while being reassured that this was a perfectly normal reaction.
Once that catheter was in place I was back at home for another 3 weeks with a urine drainage bag as a constant companion. When I went outside, I switched to a leg bag, but the constant sensation of that large catheter being felt along the entire urethra was an incredible turn-on and I was very self conscious about a very visible erection.
I had that catheter changed a number of additional times by the nurse, and a few times my wife or I did it (we are both in the medical field). I finally went back to work 5 months later... with a leg bag. I have never had an infection and I have been studied as to what I do to be so lucky not to have one.
For those that are into catheter play, I would strongly urge you study everything you can about maintaining everything perfectly sterile as you insert the catheter. That means gloves, lube, and everything. Once you touch something with a sterile glove, if it is not something already sterile, your gloves are no longer sterile. You also need to drink like a fish to keep your bladder flushed during and after catheter play.
Oh, by the way, my wife and I did experiment with cumming with the catheter in and it's a blast. When having sex, you can fold the catheter back over the penis and put a condom over it. You don't want to go into a vagina without a condom over the catheter since the vagina (even one you love) has a lot of normal bacteria and you don't need any following the catheter back into the bladder.
There's more to this story for another time.