I've had two experiences like that.
The first time, I was 15 years old, and was in a relationship with my first girlfriend. We were at the movies, and we didn't watch much of the movie. We were kissing, I was fondling her breasts a little, and she was letting me. And then, without me expecting it, she put her hand on my thigh. It was enough for me to ejaculate while moaning. She remained shocked a moment by my reaction ^^.
The second time was with a man who had been hitting on me for a while (I had almost no experience with guys yet). We were alone at his place, and he started a game where he pretended to give everything to seduce an imaginary guy... And it got to the point where he would tell what he would do to sexually heat up his "prey". After a while, I was pretty excited too. He got up to "go have a beer", and walked behind me. There he ran his hand through my hair, pressed his face against my neck, and whispered in my ear "and by the way, I think you're really hot". And just like that, in an uncontrollable and unexpected way, I ejaculated in my pants... Of course, he noticed. And I still see his little smile of victory...