Haven't read the book yet, but here's the supporting evidence I gathered from the Amazon.com preview and the authors' website and blog. (I probably missed some so please add to the list.)

• females have potential for multiple orgasms
• higher popularity of pornography with one female and multiple males compared to one male and multiple females
• female copulatory vocalizations
• male anatomy indicating sperm competition
• Coolidge effect in females
• it fits better with "fierce egalitarianism" of forager bands: female sexual exclusivity is necessary for males to determine paternity, but if all resources are equally shared, then there is little point in knowing paternity
• ETA: our closest primate relations, chimps and bonobos, have "multimale-multifemale mating systems"

On a separate note, while it seems plausible that the authors of the book are right that our forager ancestors were polyamorous, it's not clear why that matters to us in making our own choices, given that our ancestors switched over to monogamy/polygyny as soon as agriculture was invented.