Squirrels is right on.
Women typically cheat because some need is going unmet in the relationship---usually emotional. Rarely is cheating a purely physical affair with women; it starts with some emotional doors being opened and the other guy meeting whatever need her husband/BF isnt. From that point it goes to sex.
So you have a reason--some emotional need unfulfilled---and an opportunity--another man comes along who gives, or at least appears to be giving it to her. Then comes sex, and it all becomes justifiable to her (it just happened, I didnt mean to, it worked out for the best) before the guy ever finds out.
If you look at those examples (whether they are real or not), that's the common theme. No passion, opposite interests, meeting a really nice guy after being with a jerk, long-distance, and of course the classic branch-swinging to avoid a confrontational breakup or being left alone.
This is just my opinion, but women are actually remarkably loyal when they are truly happy with their man. They will move mountains for him. It's when the guy gets lazy or apathetic that these doors sometimes open, or maybe they were just a bad match to begin with.
I once hooked up with a really good female friend in college. We had dated a bit as freshmen, but never had sex, then just became good buds. So she eventually got together with a guy and became really serious. About three years after we dated we slept together on my 26th b-day. It was her driving it the whole way---I was hammered drunk, it was my b-day, and my pseudo-"gf" didnt even give me a phone call that day...so I did it. Never regretted it. Surprisingly, she didnt seem to either. We still continued to hang out like nothing happened, and she reminisced about it happily on occasion. To this day I dont really know why she did it, because her bf was a really cool guy and they are now married with a baby on the way. I think it was probably an absence thing, because he was away for the Army a lot. One of many reasons I dont believe in LDRs.