When I was about 10 or 11 and my hormones were beginning to rage, I had a constant problem with inappropriate erections. We had a baby-sitter at the time - a neighbor in her mid-teens who went to a local parochial school and usually wore her uniform when she came over to sit. I was always looking up her skirt at her panties and, naturally, this would cause me to become erect. The sitter, I soon noticed, would glimpse at my crotch whenever she thought I wasn't looking. I also noticed she would actually arrange herself on the couch in such a way that I'd be sure to see her panties, which were quite often darkened or made opaque by her arousal that would leak into the crotch.
She would often tickle and wrestle with me, making sure to rub her crotch against the lump made by my erect penis against the front of my pants. Once, while doing this, I orgasmed into my pants and the spreading stain became quite obvious to us both.
She laughed it off, but later suggested that I show her what happened. She wouldn't allow me to strip naked - she was a Catholic girl, after all - but figured, she said, that it would be okay so long as neither of us took off our clothing and actually showed our genitals.
Eventually, we progressed to mutual masturbation sessions, rubbing our genitalia through our clothing to orgasm. She said it was very arousing to her to watch the shape of my penis grow inside the soft cotton of my underpants and pajama bottoms and she told me that it made her climax when she saw my penis twitch and the material suddenly dampen, the stain spreading rapidly as I orgasmed, ejaculating my semen into the cotton in great gushes that sometimes seeped right through the material in white puddles.
To this day, rubbing my penis and ejaculating into my underpants is one of my favorite ways to masturbate and orgasm.