Humans are naturally wired to seek self-gratification. If this urge surpasses the desire to create a meaningful romantic partnership, the likelihood of infidelity increases.
Relationship fulfilment depends on the balance between the desire for self-gratification and willingness to sacrifice the desire for mutual benefit. Studies show that men are less inclined or willing to sacrifice their immediate satisfaction for long term fulfilment. In real terms, this means that when presented with the opportunity to cheat, men are more inclined to put their own needs first. Women in the same situation, are more likely to weigh up the costs and benefits before committing infidelity. This reasoning might explain why men are statistically more likely to cheat.
The self-gratification/self-sacrifice dichotomy is false. There are innumerable other factors that go into shaping relationships and their overall health.