The Monkey and the Inkpot
I'm still reading Adam's Curse by Bryan Sykes. It's going through a lot of ground related to why so many species have sexual reproduction and how that varies from one to another. I do love learning weird animal facts. I'll discover some detail about the life cycle or diet or something of this animal or that, run to Wikipedia to see what else that has to say, and then try to find a way to bring that animal or maybe just that dynamic into a story. This morning it was a tiny snippet about a tarsier in a book my son has from the library--cool animals. There's something Gollum-like about that picture on Wikipedia...
Anyway, the weird animal details in this book relate to sex--not the act, but how an individual animal's sex gets determined. In humans it's in the genes so that males and females have distinctly different chromosomes, but that's not always the case. For some reptiles, it depends on the temperature of the eggs as they develop in the nest. For a certain species of fish if you remove the one male from his harem of females, the largest female will undergo a rapid sex change so that there remains a male in the group. For a certain marine worm, the female is 10-20 times as long as the male, and the male lives inside her body where his only role is to produce sperm. Sex for them is chosen by whether the sex-less larva happen to within reach of a female's meter-long tongue (in which case it gets pulled over to the female and becomes male) or not (in which case it establishes itself in the currently worm-less space and grows much bigger, into a female).
I've read some SF with bizarre (seemingly to a human) sex and gender set-ups, but not as much as maybe there ought to be, if we weren't so blinded by what's familiar, and seldom as out there as some of these animals in this book. LeGuin's Left Hand of Darkness certainly deserves mention, and I seem to remember a story I read last year where sex was a function of age, so that the young ones were male, and as they aged, they turned female. Or maybe the other way around. I'll have to see if I can track that one down. And there was that lovely story in Clarkesworld a few years ago where the POV character was female, and she had several second-class-citizen wives, who were all male ("The Beacon" by Darja Malcolm-Clarke). Not much else comes to mind at the moment.