I think that men cheat primarily for emotional reasons, but these are often incorrectly labeled as sexual. By emotional reason , i mean there is some emotional pay off, rather than a sensual or physical sensory payoff.

Men in a relationship cheat because there's an emotional boost of feeling like having a whole other woman without their current partner / spouse knowing. The motivation for this type of cheating isn't sexual because two separate women do not increase the physical pleasure for one another, they are still two separate experiences.

Similarly, a man sleeping with a married woman does so because of the intense emotional boost of having that power over another man. Personally, my biggest fantasy is definitely sleeping with a married woman. The idea of some woman who's already married choosing me over her spouse seems euphoric to me.

But overall, men cheat for emotional or mental boosts, not increases in physical pelasure