This has happened to me twice.
The first time was with my first boyfriend. I was in college, he was several years older. We had been together for a couple of weeks, and I had sucked his dick several times. I loved it and so did he, but I had yet to make him cum. Inexperience, I suppose.
One evening, Bob was lying back in bed with his legs open and me going at it, my mouth on his cock, pleasuring him with lots of enthusiasm, if not technique. Suddenly, I felt both his hands on my head. He gently, but firmly, held me very still. Then he started slowly fucking my throat. He pressed his cock head a little deeper than I was comfortable with — just past my gag reflex — and then just a little bit more, rubbing his glans against the soft back of my throat. He pulled his penis halfway out, pushed back in, just past the gag, then just a little more, like before. He repeated this pattern for several minutes. With my mouth and throat full of cock, I remember taking mental notes: I see, so I need to allow the penis in deeper, I need to let the head rub up against the soft throat tissue, and I need to do this more slowly, rhythmically and methodically. These thoughts were in my head, and I was focusing really hard on suppressing the gag reflex, when I suddenly heard Bob groan softly, his hips froze at deep thrust mode, his hands held my head firmly, and I felt a blast of semen. Then another, and another. The shots coated the back of my throat, some of his load going straight down the gullet, and the rest spilling back into my mouth for a moment before I could swallow. I tasted ejaculate for the first time ever! I sometimes wonder what my face must’ve looked like at that moment. Lol.
I suppose that if I had really wanted to, I could’ve broken free from Bob’s hold. But I didn’t want to. I had a huge crush on him, so swallowing his seed seemed like a privilege, even if that first time I had had no intention of swallowing at all. I continued being ‘privileged’ for a whole year after that, making him cum every single time. It was a tough lesson, but somebody had to go through it. Lol!