Escapism. The marriage is definitely on the rocks --- couple it up with the bills, kids and never ending arguments and eternal fights about issues they've been battling since the beginning of time. Although we can't really question if she's still in love with her husband (and most married women would attest that cheating doesn't mean they don't love their spouse anymore), they're definitely yearning for some sort of escapism for even a short period of time.
Excitement. Their life is boring --- period. Although this reason is a bit vague and all-out I'm-so-self-centered, women use this excuse to cheat on their husbands. Romance is a very important part of a relationship and it's one of the strongest foundations of marriage. That's why when the excitement is lost (that includes sex) women might seek it from someone else.
Experience. This usually happens to women who married at an early age. There will come a point in their life that they would want to seek their other options and get a little bit too curious to other people --- and even marriage wouldn't stop that. Of course, that's if they're totally unhappy with their partner. They would seek experience outside their marriage --- usually, just sexual.
Sex. Sex is a gift to a couple after getting married. Although lack of sex doesn't follow that there's lack of love, some married women would confess to the fact that they're lack of satisfaction in sex with their partner may lead for them to cheat. Usually married women only seek sexual relationship outside marriage. And that would follow that they can be a little emotionally linked too.
Satisfaction. But what most married women would agree on is that they cheat because they're dissatisfied --- with everything. They're unhappy, they're looking for something new, someone that will make them feel alive and inspired once again. Satisfaction usually follows happiness and when it's not there, then she'd definitely find it somewhere else.