Well mine has two parts, but I was pretty young. This is going to be very long please forgive me.
The first is when I was about 6. My mother is a doctor and did not believe in shielding the truth from children so only a bit before, as soon as I had learned to read enough to do so on my own, she checked out 8 or so semi-child friendly books about how pregnancy works and did the birds and the bees explanation herself. She would come to regret that decision as I then explained the birds and the bees to all my friends who were curious lol but moving on...
So I already knew how all that worked and my neighbor friends who were boys my age knew that I knew as I had explained to them. So they were not shy. Our backyards connected into a kind of densely wooded wild area as they were both new houses and no one had bothered making the backyard pretty. So it was dirt and trees and weeds. They had a trampoline that we usually played on and if they had to pee they would just hop off and pee on a tree, or sit on the edge of the trampoline and go through the springs like it was a toilet if they had to go number one AND number two.
Now at this point I wouldn't really call it a fascination, just opening my mind to the fact that you could in fact pee outside the toilet. At the time I think I was actually jealous and annoyed at them because they only had to pull down the front of their pants and I had to run all the way behind a tree, pull my pants all the way down, and squat and it was very troublesome. But this paved the way for when I "realized" what I liked.
Later in life I am not sure how old I was, perhaps 10 or 11? Maybe 9? But one of my parents friends had fallen on rough times and needed a place to stay while they found a new place to live. They had a daughter around my age and we played together often since she was always around. It was summer and while the pool was close we couldn't always go because it was busy and we kept having outbreaks at the pool because of overcrowding. So we often played in our swimsuits in the sprinklers and we would be out there for hours.
Naturally when you are outside for hours you drink a lot, and then you need to pee, and the water in sprinklers is very very cold. Cold enough that even on a really hot day you have to stop for awhile and let the sun and hot concrete warm you up as you shiver, and the toilet seat is very unfomfortable when it's that cold, as is the air conditioning inside.
So when I told my friend I had to pee she gave me a look and said "just pee out here." I told her that pulling my swimsuit off and on would be a real chore since it was soaking wet and she told me not to take it off. "Its wet anyway." She had us sit on warm dry cement because it was more comfortable than sitting where the cold sprinklers would spray us while we were sitting still and make us super cold, and we made pee circles on the cement.
I did not really know how being turned on was supposed to feel, i was not interested in boys at all then, but that was my first sexual experience. The cement was very hot and felt wonderful against my swim bottoms and the pee was also so very warm as it semi-pooled around my bottom and trailed down my legs a bit especially when the sprinklers were so cold, and the hot cement made the pee even hotter than normal and the cement more bearable to sit on.
From that moment I was hooked. We drank more and peed all over the cement and the dry brick wall which was very warm as well. I felt so naughty doing this I wanted to play in the sprinklers every day so I could do sprinkling of my own. At first we peed about equally but soon I was even more enthusiastic than her. I felt so naughty peeing where everyone could see me in our small front yard since it was a busy street and knowing they didn't know what I was doing. Soon I was playing in the sprinklers alone so that I could be more naughty.
I hated my brother because he had a mental disorder and would beat me and my parents never did anything about it because of his disorder and I was too young to really know what was wrong with him so I would bring his toys out with me and hide them at the side of the house where no one would see. Then when I had to go when playing in the sprinklers I would go over to them and pee through my swimsuit onto them as revenge.
After a bit I got more extreme and began peeing inside. I would go to use the bathroom and instead squat in the shower or trash and pee there. And of course my brother was gone at therapy a lot so I would sneak into his room and have sneaky pees everywhere.
In the end my mother discovered a dried pee spot of mine in the guest shower. They blamed it on my brother of course so nothing at all was done but it made me nervous and I realized I didn't know how to cover my tracks well. They also thought he had been peeing in his new room since he had peed in his old room all the time when he was a bit younger because he didn't know better. It smelled like pee so I stopped and I felt guilty since he is my brother still so I used my allowance to buy cleaners and volunteered to clean his whole room. I wanted to do it secretly so my parents wouldn't praise me but I knew if I was caught they would know it had to have been me and i really disnt want to get in trouble haha. I still didn't really know it was sexual I was too young all I knew was it felt good.
I stopped for a few years then but when I went into highschool and started learning about sex feeling amazing and fetishes from my school mates I realized and started seaching up pee stuff. I did pee more in the house then but I tried to be more sneaky. A few more times my pee vandalism was caught but the finger was never pointed at me as there were more small kids by then. I also tried to make sure it would be cleaned up by me in secret occasionally but there are still some very destructive pees in the storage room I know they haven't found yet from when I was testing my limits on what turned me on about this fetish, but I live on my own now and have for some time so I know it won't fall back on me haha.
Sorry this was so long...