The Narcissist feels (and thinks at an unconscious level) as if the person denied him sex must find him unattractive, or she think something is wrong with him. If this happens in a relationship he likely feels the same, but feels as if something about him must be worse since now his partner doesn’t want to have sex all the sudden.
When he feels this way he will likely get upset and be unpleasant, mean, tell you how you are the problem and that he doesn't want to have sex with you anyways, and act in behaviors similar to what I described.
He will behave this way because his false self that was developed at a young age tells him he is everything great and that he is never wrong, and that he is everything great that he deep down feels like he is not.
The Narcissist truly believes the false-self to be who he is while not knowing that all the false self really is, is a false persona created to prevent him from feelings the horrible feelings that he felt as a child.
He has lost his true identity and now is just a shell of a person who is nothing but a walking and talking person enslaved to a defense mechanism.
After the Narcissistic abuse or manipulation ends you will either be manipulated into having sex with him, or say no and not have sex with him.
If you hold your ground and deny sex the Narcissist will more than likely leave, and he probably will blow up your phone texting you, trying to get you to give in a have sex.
If you continue to deny him he eventually will see that nothing he does work and give up and move onto someone else.
This is one possible outcome. Another possibility (if you are both not serious) is that the Narcissist leaves you alone and that's that. If the Narcissist has more options that he finds equally or more appealing then he will probably move on with no issue as in his mind he is a Good, and you are an idiot who is denying sex from the worlds greatest person.
In general of you deny sex and hold your ground, eventually the Narcissist will leave you alone and find someone else to abuse and manipulate to fuel his ego