Sometimes a man realizes that if one woman cannot satisfy him, it isn't necessarily her fault, and to simply tell her would be to just needlessly hurt her.
Then again, perhaps some men also realize that if they forced themselves to be strictly monogamous they could probably do it, but it would generate stresses and resentments against their mates that might be beyond their control. You can force yourself to do or not do something, but you cannot force yourself to feel a certain way.
I'm sure women do the same thing, but in other areas of life.