It's mostly a US thing, I've seen a huge divide about marriage between old Europe and the US that's due to influence of puritanism. Only 20% of men in the US cheat, that's a relatively low figure.

The moralists also conveniently ignore that the vast majority of men is not very attractive. Cheating would take a lot of effort for them. In contrast, I knew some men who were attractive enough they had to fend off women almost daily. For them, not cheating takes a lot of effort. The bottomline is that most of the 5/10 guys who say "I would never cheat" would do so the very next day if a 9/10 woman suddenly got interested in them, which is exactly what happened in this case.