My first time naked with another is perhaps a little unusual, and happened when I was 9 years old, and I had a girlfriend who was also 9 years old.
One morning in the summer of 1979 I was at her house, and her mother was there also. Exactly how it happened is lost in the mists of time, but somehow I ended up being invited to join my girlfriend in a bath her mother was going to run for her, and I accepted. I recall climbing the stairs with my girlfriend and we undressed shyly in her bedroom before crossing into the bathroom. I was completely accustomed to being nude at home with my mother, but being nude with someone else, especially a girl my own age, was something completely new. She was completely amazed at seeing a penis for the very first time, and for me it was the first time I had seen a hairless pussy.
While waiting for her mother, my girlfriend jumped onto the toilet, and sitting with her legs wide apart, she urinated and put herself on full display. She laughed and giggled at her own outrageous behaviour, watching my reaction to what I was seeing up close. I believe it was this specific incident, more than any other, that gave me a sexual interest in female urination. At that age it was very formative. When she was finished, I too stepped forward and I urinated into the toilet as well, and she obviously enjoyed seeing it. I seem to remember her mother entering the bathroom while I was halfway through it.
After her mother ran the bath and stood back to supervise, my girlfriend and I stepped into the water and washed ourselves. I recall washing my girlfriend's back and she washed mine. Beyond this, my memory of this shared bath is patchy and unclear, but I do remember getting an erection at some point. My girlfriend giggled at it, and she tentatively prodded it once or twice with her finger. It was all innocent fun, and yes, a little bit sexual but in an age appropriate and educational way. There was certainly no harm in it, and I remember my girlfriend's mother explaining to her daughter why I had an erection.
My girlfriend came to know my penis very intimately when we were both 16 and she had her first time with me. At that age we had obviously developed physically, and she had a really nice little figure by then, and she was great fun in bed. When we both went away to different universities at 18, we drifted apart romantically but we remain friends to this day.