This news story carried on Reuters reports that high levels of a certain kind of estrogen in women leads them to shop around for better mates even when they're in an established relationship. The suggestion comes from within their body chemistry.
From an animal standpoint it makes sense. A fertile, attractive female can attract better and better males, leading to offspring with theoretically better chances for survival and success.
Many of the conscious decisions humans have made about mating behavior over the years have dealt with known and logical indicators of status and success. This recent study simply provides further proof that the mechanisms are more automatic than many people may realize or wish to admit.
Guys, if you get dumped by that hot chick, remember two things: if you got to be there at all it's a compliment, and she didn't really have control over the decision to move on. Oh, and you're genetically inferior, so try to stay out of the way, okay? The hormones don't lie.