because its "safe". They can fool around but when it starts getting serious, they can always fall back on the "I'm married/ have committments/the children are too young" routine. The wife doesn't know, so they figure..no harm done.

And by the way, its not just men that do this and its not just men that find the need for variety, that's an old excuse to explain away the responsibility. Its NOT human nature for men to stray..that's just plain silly.