Writing about sex is tricky, because you immediately are set upon by all manner of ideologies. Every religion, political ism, or psychology has a stance, usually amounting to different flavors of "don't do that; that's not right." Since Darwin, it's been almost a sport to use evolutionary biology to those ends: x isn't found in nature, y is aberrant, caused by disease or choice, and has no evolutionary benefit, etc.
Olivia Judson, an evolutionary biologist, adopts the persona of Dr. Tatiana, and accepts questions from all manner of species about the amazing variety in their sexual and reproductive lives. Say you're a slime mold, and are confused about how it works to have at least eight different sexes, or you're an ancient asexual that has done without males for 75 million years, what's up with that? Dr. Tatiana very deftly and with a minimum of jargon relates how the sex thing works, why it works, and what questions biologists have been asking over the years. She keeps the politics to a minimum, instead charting the eye-opening variety out in the various kingdoms.
Politics are inevitable. Biologists fall prey to cultural ideals that may or may not have anything to do with natural selection. Just because an idea sounds good, doesn't mean it's actually the case. Take the whole "men screw around and women really don't" theory. Everyone from some brands of feminists to most churches hold this view, and it was bolstered by the biologist's notion of "eggs expensive, sperm cheap." Females will guard their eggs; males will behave like fire-hoses. Well and good, and self-evident to tons of different types philosophies.
Unfortunately, not actually borne out by data. With the advent of DNA testing, it was found that female promiscuity is more or less the norm, even in species that create what look like monogamous pair bonds. Also, the ladies who stray have more successful children than the ones who don't. Dr. Tatiana lays this out in the opening section, which totally blew my mind, and it only gets weirder and more interesting from there.
She doesn't, however, take the fact of female promiscuity to advocate one thing or another, which is totally sweet. She also doesn't talk much specifically about human sexuality, which is also sweet. I'd rather be stunned by the fact that porcupines have much more intercourse than is necessary for reproduction, that armadillos as a matter of course produce a litter of four clones out of a single fertilized egg, that the penis has evolved no less than seven different times in different classes of animal and on different parts of the body. (Spiders go for the head. There's a bad joke in there. You have until the end of the class to come up with one.)
There are some things I would have liked her to address with more depth: wherefore female pleasure? No one knows, eh? Any ideas? The frothy airiness of the format doesn't really lend itself to in depth discussion, but it probably solves more problems than it creates. There's a huge bibliography should one be interested in reading more.