For a counter theory, try reading Sex at Dawn. It's far from perfect. The authors accuse "the establishment" of picking and choosing evidence to show that men stray to spread their genetic material, while women are glorified prostitutes: seeking shelter, food, and security from men, while trying to obtain superior genetic material by cheating. They then turn around and pick and choose their own evidence to support the theory that humans aren't and were never genetically monogamous, and that we stray because we are trying to force monogamy as a fundamental human condition. Nevertheless, the evidence they do present is compelling enough to show that monogamy isn't fundamental to humans or our closest relatives.

For an even better read, try Mating in Captivity which talks about sex and intimacy from the point of view of examining couples whose sex life has died away. This is better place to start if you want to understand cheating.