If you're in search of ideas for truly alien sex, pick up SEX IN THE SEA, by marine biologist Marah J. Hardt. She begins with the quest for a potential mate, not always easy to find in the vastness of the oceans, and continues through courtship, mating, fertilization, and birth or hatching. Some sea creatures travel formidable distances back to their birthplaces to reproduce, while at the opposite extreme others (oysters, coral, etc.) spend their adult lives stuck in one spot and somehow have to get their gametes together without moving from that spot. Some animals change sex when they mature, while a few species can even flip their genders back and forth multiple times over their lives. There are species with two or three distinct types of males, one of which looks like a female in order to sneak past dominant macho-style males and mate with real females. Some male animals have penises longer than their bodies. There's even one that has sperm cells longer than its own body! Female right whales can have intercourse with two males at once, one on each side. A certain segmented, sand-dwelling worm has multiple penises or vaginas, one for each segment. Hardt says they mate like a zipper closing. Squids and octopuses use a specially adapted arm to place a sperm packet inside the female. There are creatures that detach their penises like darts. Hermaphroditic flatworms don't copulate in the "normal" way but stab each other with their organs to inject sperm anywhere in the mate's body. The males of some species of fish attach themselves to the bodies of their much larger mates and atrophy into mere sperm-dispensing appendages. One species takes this process even further, with a female hosting numerous tiny males inside her body. In one kind of shark that bears live young, stronger fetuses murder their weaker siblings in the womb.
Imagine what marriage would be like on a world where the dominant species reproduced like angler fish, with the husband a parasitic attachment to his wife. Consider the dramatic possibilities of sibling rivalry among intelligent beings who know their potential brothers and sisters were eaten in the womb. Sexual politics in a species that reproduced externally, like most fish and amphibians, would be quite different from the status quo in our culture, where the burden of carrying the young inside the body falls on the female. One of Fredric Brown's humorous short-short stories features a man who falls in love with a mermaid and agrees to be transformed into a merman so they can marry. After the change, he's horrified to learn that merfolk mate like fish, by spawning into the water instead of copulating.
Suppose a human hero fell in love with a member of a gender-fluid alien race, able to change sex back and forth depending on environmental cues such as the sex of his/her mate (like some fish). Marion Zimmmer Bradley creates such a race, the chieri, in her Darkover series, and one of the human characters in THE WORLD WRECKERS faces that very challenge.
Every chapter of SEX IN THE SEA offers similar thought-provoking oddities. Written in a breezy, slangy style, this book is both fun and informative.
Margaret L. CarterCarter's Crypt