"Surprisingly, research and anecdotal data shows that infidelity does NOT usually happen because of problems in a marriage." read more
They list a few reasons, but the one that I can relate to is this one.
"Feeling Attractive - If there is any reason for female infidelity that I have heard and seen the most, it is the woman's experience of a sense of attractiveness from the attention of men other than her husband."
I accept all of that information.