The guys that cheat either: weren't planning to, but the opportunity presented itself and did cheat, or they are just interested in sleeping with many women.
The first type made a mistake and realize it. They might learn from it.
The second type are in a relationship not for sex but for a variety of other reasons. For companionship, or they can't be intimitate with women they actually love, to feel safe, for prestige, to look more attractive to other women because the look more stable and dependable, other women notice them more especially women interested in sex, they like sex with lots of women, etc....
This sort of thing is probably going to happen more and more often for both men and women. Mainly because dating rituals are no longer really practices, and serial instant relationships abound.