This amazing non-fiction book discusses "Diversity, Gender, and Sexuality in Nature and People" and shows how blind and stupid the religious fanatics are when they claim that homosexuality is unnatural. It's perfectly natural in that we see it throughout nature, where gender is even less of a binary matter than it is typically perceived as being in humans. Joan Roughgarden is an ecologist and evolutionary biologist who has written several books on the topics, and in this book she explores diversity in gender and sexuality among fish, reptiles, amphibians, birds, and mammals, including primates, as the blurb says.
She takes issue with sexual selection, which has been a tenet of the scientifically established Theory of Evolution since Charles Darwin himself published The Descent of Man, and Selection in Relation to Sex way back in 1871. I disagree with her on that score, and have to point out that the book is in places, rather didactic. She has a soap box and she's sticking to it, but on the other hand, being a transgendered mtf herself, she does have an inside track! However, anecdote isn't the same as data, so beware of taking everything she says at full scientific value.
It's important to keep in mind that this is a book expressing a PoV, not a science paper, so it's written in layman's terms and a lot of it is not established scientifically, but I did not read it for that, I read it precisely for the diversity portions, and those were highly informative and quite entertaining. Note also that Richard Dawkins's popular books are, for example, written in precisely the same way as this, so there's nothing substandard or unusual about this style of writing.
While I would take issue with her theistic evolution viewpoint, I do every much enjoy her writing, and I recommend this educational book highly. It's a pity that those who most need to read and learn from it will doubtlessly dismiss it out of hand.