Narcissists cheat on their spouses, commit adultery and have extramarital affairs and liaisons for a variety of reasons which reflect disparate psychodynamic processes:
1. In the quest for narcissistic supply, the somatic narcissist resorts to serial sexual conquests.
2. Narcissists are easily bored (they have a low boredom threshold) and they have a low tolerance for boredom. Sexual dalliances alleviate this nagging and frustrating ennui.
3. Narcissists maintain an island and focus of stability in their life, but all the other dimensions of their existence are chaotic, unstable, and unpredictable. This “twister” formation serves many emotional needs which I expound upon elsewhere. Thus, a narcissist may be a model employee and pursue a career path over decades even as he cheats on his wife and fritters their savings away.
4. Narcissists feel superior and important and so entitled to be above the law and to engage in behaviors that are frowned upon and considered socially unacceptable in others. They reject and vehemently resent all limitations and conditions placed upon them by their partners. They act on their impulses and desires unencumbered by social conventions and strictures.
5. Marriage, monogamy, and child-bearing and rearing are common activities that characterize the average person. The narcissist feels robbed of his uniqueness by these pursuits and coerced into the relationship and into roles – such as a husband and a father – that reduce him to the lowest of common denominators. This narcissistic injury leads him to rebel and reassert his superiority and specialness by maintaining extramarital affairs.
6. Narcissists are control freaks. Having a relationship implies a give-and-take and a train of compromises which the narcissist acutely interprets to mean a loss of control over his life. To reassert control, the narcissist initiates other relationships in which he dictates the terms of engagement (love affairs).
7. Narcissists are terrified of intimacy. Their behavior is best characterized as an approach-avoidance repetition complex. Adultery is an excellent tool in the attempt to retard intimacy and resort to a less threatening mode of interaction.
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”.