Between watching the Vancouver Olymipics this weekend, I'm reading a book by Andrew Trees called Decoding Love. It's subtitled "Why it takes twelve frogs to find a prince and other revelations from the science of attraction."
There are loads of insights, which I am still trying to digest. I'll add that to my new book on evolutionary biology with has some fascinating thoughts about why people cheat. Apparently according to Trees and Bunson (the other science book), we are collectively 76% likely to fool around on our partners. Women are 20-40% likely to fool around.
The chemistry of attraction takes 2-3 years to wane before we start to feel "neutrality" (isn't that a scary word) about our partners. One of the most interesting facts that Tress presents is the insight into women's fantasies. Women who fantasize about other men are the ones most likely to cheat. However if you encourage a range of creative fantasies with your sweetie it also keeps the relationship fresh and interesting. You may run the risk of her looking at the next guy, but you keep her heat up for spin offs in your own bedroom.