Some people are never going to be faithful in a relationship, it's just not in their DNA, but I have found that it is not the case for most people. Studies on adultery point out that men and woman cheat in a relationship for totally different reasons. For men, its simple for the sex. Their spouse or girlfriend are not fulfilling the man's sexual needs, so they find someone that does. For woman, it's much more complex than just sex, they tend to be looking for their soul mate, that person that just knows and understands how they feel. It's a much more internal motivation than an external one than just for the sex. I would guess this gal was looking for someone that finds her attractive and wants her, something her husband no longer provides.