Apologies in advance for the wall of text. But I love reading about people's experiences, so it's only fair to share mine for others.
I had one done in my early 20s at the Urologist. I had been having some pain in my bladder after voiding and my GP referred me to the urologist for follow-up. I show up and am the youngest person in the waiting room by probably 40 years. The receptionist checks me in and shortly thereafter a nurse leads me to a bathroom to produce a urine sample, then to the exam room where she did a quick set of vitals. Afterwards I waited on the exam table for the urologist. The doctor was a gruff, middle-eastern man in his 60's. He introduced himself, we discussed what was going on and when I would feel the pain. He had me lay back and lower my pants and underwear below my knees which was an awkward maneuver where I had to lift my pelvis up to slip my pants and underwear down as he was in the room donning gloves. He did a detailed genital examination but spent most of the time palpating and pressing around and on my bladder. Finally he said he'd like to check my prostate and had me roll on to my side and bring my knees to my chest. He applied a copious amount of cold lube to my anus, told me to breathe out, and pushed his finger inside of me. He twisted his fingers around, and probed my prostate for about 20 seconds before quickly withdrawing and saying everything felt normal. He gave me a couple tissues to clean up with and stayed in the room finishing up his notes. The nurse came in to tell him there was a small amount of blood in my urine. After I finished, we discussed findings and he recommended we do a cystoscopy as a next step. He offered two options, the first was we could schedule another visit where there would be some anesthesia, but I would need a driver. The second, which I chose, would be to do it immediately, with no anesthesia.
The nurse led me back to a different room, much larger with more equipment including an exam table with stirrups. She instructed me to empty my bladder in the adjoining water closet and undress completely from the waist down, then wait on the table while she fetched supplies. She returned shortly with a rolling tray topped with a large saline bag and other equipment. She positioned it next to the exam table, hung the saline up, and gathered additional equipment that was in the room. At this point, she had me lay back and positioned my legs up on the stirrups, exposing me completely. She prepared some betadine in a tray, apologized for the cold, then thoroughly cleaned my penis, especially focused on my slit. Then she opened up a sterile surgilube syringe, placed the tip into my slit, and slowly injected lubricant into my urethra. She then placed a small spring clamp, pinching the head of my penis shut to keep the lube in. She covered me and said she'd be right back with the Urologist to get started.
I laid there in the quiet, staring at the ceiling and equipment and wondering what I had got myself into. Shortly thereafter, there was a knock at the door, and the Urologist entered with the nurse. He positioned himself between my legs, and uncovered me while explaining what was about to happen. The nurse was positioned to my side and was to control the saline flow and video equipment. The Urologist uncovered the flexible cystoscope and squirted a large amount of surgilube along its length. He removed the clip from my glans, then held my penis in one hand. With the other, he picked up the cystoscope, positioned the lighted tip at my urethral opening, and slowly advanced the camera inside of me. It was such an intense experience and I couldn't decide if I should watch the video monitor or the cystoscope disappearing down my penis. I couldn't believe something that large and long could slide down inside of me. The nurse was still standing to my side, one hand on the controls for the saline, but she was staring intently at my penis and watching the Urologist manipulate the cystoscope further into me. I'll pause here to say I was not physically aroused at this point at all. In fact, its miraculous there was even any penis for the urologist to hold onto at all. But this whole experience was erotic in a way that's difficult to describe. Being exposed, with my genitals in focus, in front of both an older woman and man. Total loss of control as both the Nurse and Doctor manipulated my penis and introduced sensations I had never experienced before.
He would stop occasionally to let the saline flow open my urethra. We came then to the bladder opening. He paused and advised me to take some deep breaths, then when I was breathing out, pushed the camera through the sphincter and into my bladder. The nurse loosed the saline, filling my bladder, and the urologist twisted the camera round to see the entire lining. After a few moments looking around, he withdrew the cystoscope while some saline dribbled out along with. The nurse assured me this was okay and was just saline from the flush. The pad I was on caught any spilling from withdraw. She took a minute to clean me up with some damp washcloths while the Urologist was placing the cystoscope down, disposing of gloves, and washing his hands again. The nurse positioned my legs off of the stirrups, covering me again with a sheet, and sat me up. The Urologist returned to discuss findings (nothing abnormal), and gave me a prophylactic antibiobic with some water. At this point, the Urologist said they would leave me to get dressed and empty my bladder (which was very full). Provided I could go, I didn't have to stick around and could check out and leave. I hopped down from the table, and walked over to the toilet. I had a little difficulty going, but managed to with some intense burning through the length of my urethra. I took this opportunity as well to snoop in some drawers just to see what there was to see. I finished getting dressed, then checked out with the receptionist, no follow-up required.