Narcissists often prefer masturbation to sexual intimacy because they don't really empathize, so they don't gain anything from emotional connection with a partner. If there's a touch of sadism, they might get something out of degradation, but that's about it.
So they're there, but they don't really see you. They tell you what you want, and don't take feedback. It's all about them and what they want. If you enjoy something, they may lose interest in it, because then it's about you, so they have to up the ante and find the next thing that makes you uncomfortable, or at least unsatisfied.
Sex with a narcissist is bad, and that's probably good, because they have enough control over you without the sex being another card to play.