As with lying, cheating is common with Narcissists. They tend to have an intense need to feel adored and desired by others and they feel entitled to partake in affairs to meet their needs. They often get bored with being limited to a single partner and may end up with multiple mates to keep a steady supply of adoration and variety at hand. They also use sex as a means of control of their partners and a way to keep others in “relationships” with them.
As practiced liars their escapades can go undetected — or their victims may suspect them at some level but not question them due to having been gaslighted or otherwise controlled or demoralized by them to such an extent that the cheating is allowed to continue.