I have been thinking of that question and can link some early experiences growing up which I am confident sowed the seeds of my growing willingness to expand my sexual horizons beginning in my 50s and continuing today.
At age 13 or 14 my good grade school buddy had major back surgery including a full body cast.
I remember visiting him at the hospital and asking about the pee bag hanging from the table which was a new sight for me.
What I recall also is he told me he was getting used to it this tube called a catheter (and showed me everything) and that it actually felt good to have it in. That continued for the rest of the summer while he was healing from the back surgery. hmmmmm.
Shortly after that I was playing a game called Kerplunk that used thin plastic picks about 6 inches long which were put into a plastic basket and filled with marbles. Object was to remove the picks one by one without causing the marbles to fall to bottom- but I digress.
So one night I got wondering what it would feel like to stick one or those picks into my little cock which was just beginning to emerge into puberty.
Using vegetable oil as I recall I must have enjoyed the experiment because it ultimately triggered a spontaneous and one of my very first orgasms with ejaculation. I shared this little discovery with a few buddies in mutual play as it seemed I and we had near perpetual erections and multiple Jack off sessions a day back in those days.
Next at age 16 I was accepted and trained as Volunteer Assistant Orderly in a program at local hospital where I worked for over a year. I spent Friday nights in ER and Saturday’s in the Kidney and Urological ward. Believe me, that 16 yr old saw more than I should have been allowed to see (but I wanted to become a doctor I thought) including a whole lot of foley cath insertions m/f as well as cath care, flushes and removal. I was both intrigued and wondered how painful it must have been for the patients based on some reactions I would often see.
I am convinced those early experiences triggered something in my brain that caused me in my 50s to begin learning about and playing with my urethra using sounds and catheters for sexual arousal and enjoyment. I think I was determined to change my early perceptions of a painful and terrifying experience with sounds or catheters into just the opposite- Pure (or mostly) pleasure. Mission accomplished.