How do I explain to him why it takes so long for me to ?

My boyfriend really is a great guy, truly. He's eager to please and wants to learn and be a great lover, but he's watched so much since he was young, and that's tainted his view of and what he thinks women like. He thinks the women in are actually c*mming that quickly and wonders why all the women he's been with, "hold it in for so long," as he puts it. I've tried explaining to him that while some women may actually be lucky enough to finish that quickly, the women in are actors, and the vast majority of us simply can't. It's just how we are. It's like I have to totally retrain him lol. How do I explain this to him better?