Narcissists cheat on their spouses, commit adultery and have extramarital affairs and liaisons for a variety of reasons which reflect disparate psychodynamic processes:
Broadly speaking, there are two types of narcissists, loosely corresponding to the two categories mentioned in the question: the somatic narcissist and the cerebral narcissist.
Narcissists are misogynists. They hold women in contempt, they loathe and fear them. They seek to torment and frustrate them (either by debasing them sexually – or by withholding sex from them). They harbor ambiguous feelings towards the sexual act.
The somatic narcissist uses sex to “conquer” and “secure” new sources of narcissistic supply. Consequently, the somatic rarely gets emotionally-involved with his “targets”. His is a mechanical act, devoid of intimacy and commitment. The cerebral narcissist feels that sex is demeaning and degrading. Acting on one’s sex drive is a primitive, basic, and common impulse. The cerebral narcissist convinces himself that he is above all that, endowed as he is with superior intelligence and superhuman self-control.
Still, sex for both types of narcissists is an instrument designed to increase the number of Sources of Narcissistic Supply. If it happens to be the most efficient weapon in the narcissist’s arsenal, he makes profligate use of it. In other words: if the narcissist cannot obtain adoration, admiration, approval, applause, or any other kind of attention by other means (e.g., intellectually) – he resorts to sex.
He then become a satyr (or a nymphomaniac): indiscriminately engages in sex with multiple partners. His sex partners are considered by him to be objects – sources of Narcissistic Supply. It is through the processes of successful seduction and sexual conquest that the narcissist derives his badly needed narcissistic “fix”.
The narcissist is likely to perfect his techniques of courting and regard his sexual exploits as a form of art. He usually exposes this side of him – in great detail – to others, to an audience, expecting to win their approval and admiration. Because the Narcissistic Supply in his case is in the very act of conquest and (what he perceives to be) subordination – the narcissist is forced to hop from one partner to another.