Q: Why do narcissists hold sex and affection from you
A: Narcissists withhold sex and affection as a means of controlling you. If you said or did something they didn’t approve of, they withhold sex and/or affection as a means of punishing you.
Some narcissists also withhold as a means of “torturing” you, making you question them repeatedly or beg them ( not just for sex, but to understand why they are doing it- you will likely not get an answer unless it is further berating).
Remember that all attention-positive or negative- is a form of Narcissistic Supply so even if you react negatively or angrily to their withholding, they still receive Narcissistic Supply.
Having you beg them and give them attention with frequent questioning are forms of Narcissistic Supply.