I think that many women (more than most would admit) are driven by their need for the 'thrill' and 'excitement' that sex with a different person would bring. They are just more discreet about it. We all know that it's a given that women are more conscientious when it comes to lying (by omission or otherwise); for one - rarely would a woman forget what she lied about, as many men have.
Women are just as likely to cast the net as the men do, it's just that perhaps women are afraid to do so for fear of being labeled as a 'whore' or a 'slut'.
To be fair, many women enter a relationship with the hopes of it progressing into something deeper, whilst many men simply take it a day at a time and leave their options open in the meantime. I guess with marriage, the commitment on the guy's part IS there - but basically, a man needs more than just stability and security in their lives; unless of course - they have sown their wild oats enough to know that the grass was actually brown and painted green over on the other side. Many women settle into a comfortable state of being after marriage, they let themselves go, and simply relax in the knowledge that they have 'secured' their man. They stop making an effort. Perhaps this is why many men stray more than anything else... when there's nothing to look forward to when they come home, why would they want to?
Women, on the other hand - perhaps stray because of a need to be 'wanted' and 'desired', things that her husband conveniently forgets to constantly assure her once he has placed that ring on her finger. A hug, a kiss, a simple gesture of appreciation would make the big difference to a woman. Those little acts that many guys take for granted - translates into love, and the feeling of being loved - to a woman.
It's not rocket science.