I had it happen to me only one time when I was 18. Was having an intense kissing session with my girlfriend at that time. I felt myself get really hard in the jeans I was wearing and before I knew it, I exploded with a large cum shot that made a big wet spot on the inside of my right thigh. I don't think she ever knew I had done it and I never said anything about it to her either.
When I was around 6 or maybe 7, I had one climbing a tree, my legs wrapped around the trunk, no ejaculate, but a feeling I repeated time and time again. To say the least, I was a climbing addict. LOL Still am.
In history class i used to sit opposite this really hot girl who always war short skirts. One lesson i sat there with a raging boner fantasising how it would be to kneel between those beautifull legs. My thoughts got a bit carried away and before class was finished i had made a nice mess in my undies.