This is a story I originally posted elsewhere, in much shorter form. I'm planning to continue and expand it; feedback is very welcome.
Ruairí is the fwb of an fwb, sent to you to be introduced to urethral play. The 28 year old ginger otter lies with his ankles and wrists tied to the bed. You've raised the head so he can easily watch. He looks at you with apprehension and excitement.
Your gaze sweeps down from his eyes, over his muscular hairy chest and abdomen. His semi-erect cock surrounded by a trimmed bush. It twitches as he sees you looking, the foreskin pulling back a little and revealing your goal.
You take an antiseptic wipe and clean the outside of his penis, causing it to become fully hard and revealing the whole shiny purple head. Pulling down on the shaft, you wipe the inside of the foreskin and underside of the pronounced ridge of the head.
With a fresh wipe you clean his piss-slit and the rest of the head, making sure to tease his peehole a little. His cock throbs as you release it; a small drop of precum glistens at the opening.
Looking across your trays of implements you decide to begin this first invasion of his urethra with a thorough lubing. The rounded end of the syringe is pressed hard into his meatus as you spread the hole with your other hand.
You push a little on the plunger and let out a little lubrication for the opening. The tip slides in slightly; our Ruairí is blessed with a hungry hole. He gasps as you push the rest of the lube and massage the bulging ridge of his urethra to spread it within him.
As you remove the syringe a blob of lube oozes out, just where you need it.
Based on the size of his peehole and the visible thickness of his urethra you think he might be able to start with a bigger than usual sound. You pick up a 8 mm straight sound and watch his eyes widen.
The 8 mm tip slides around his piss-slit in a satisfying way as you tease a little. Following the curves down from the tip of his cock towards the hole, pressing slightly but stopping before it slips in. His cock now twitching wildly, you control it with your other hand.
You press the sound gently against his peehole. You rotate it a little. The tip is swallowed up immediately. You were right about the hunger of his hole. Ruairí grunts at the new sensation. He tries to move his cock away from the invasion. He cannot.