Re-watching part of Animal Planets MOST EXTREME "Breeders" episode reminded me of the many different reproductive arrangements here on our own planet and how they might translate to alien cultures. Admittedly, the most extreme breeder on Earth, the tapeworm, wouldn't make a very interesting alien, because it's basically all sex organs with no brain. (Resisting temptation to make a joke about men.) Number two on the list, the naked mole rat, however, has possibilities. This subterranean mammal lives in a termite-like society. One female produces all the offspring (mating with her brothers and sons) and prevents the other females from breeding. Terry Pratchett adapts this system for a society of intelligent beings in his "Wee Free Men" series. The vast majority of his pixies are male. They dwell in beehive-like mounds, each ruled by a matriarch who is married to one of the older men. She normally has only one daughter among her myriad children; the daughter, upon coming of age, migrates to a different colony to marry one of their males and become the mother of that colony's next generation.
A wolf pack comprises basically one extended family, in which only the alpha male and female produce cubs. Since this restriction would limit possibilities for romance, my werewolves don't behave this way. One of Tanya Huff's "Blood" novels, though, features a werewolf pack in which only the alpha pair is allowed to breed. A custom of that kind could make a provocative plot hook if a young male wanted to mate and therefore had to leave the pack to strike out on his own.
Imagine an intelligent species with a social structure like that of an elephant herd. The basic unit consists of related females and their children, dominated by a matriarch, with adult males relegated to the periphery. A human man who fell in love with a female alien from such a culture would have to make quite an adjustment to fit in.
While I wouldn't really want to live like a wolf or an elephant, I've often envied the kangaroo's reproductive biology. Imagine the ease of giving birth to an embryo-size infant who then crawls into one's pouch, compared to the difficulty of bearing nine-pound babies. (Two of my four sons reached the nine-pound mark.) How convenient a working mother's life would be if she could just carry her baby everywhere in a marsupial pouch, where the little one could feed himself or herself at will. Moreover, a kangaroo is one of several animals that can temporarily halt the development of an embryo, effectively putting it into suspended animation, until conditions are favorable for pregnancy. A very handy trick!
Suppose an intelligent alien race had estrus cycles like many animals? The Vulcans of the Star Trek universe, whose males periodically undergo "pon farr," have monogamous marriages, but that need not be the custom for all estrous species. Ursula LeGuin's LEFT HAND OF DARKNESS, of course, is well known for its hermaphroditic aliens who are asexual except during "kemmer" (estrus) and, during their fertile phase, become either sex at random. Jacqueline Lichtenberg's MOLT BROTHER includes reptilian aliens with the familiar two sexes and a heat cycle. When a female goes into heat, she invites several males to mate with her multiple times over the span of her estrus. In such a culture, would the female (not having a spouse) receive help in child-rearing from her siblings?
Consider fish: Some change sex from female to male according to environmental factors. Others, such as certain angler fish, have such a wild disparity between the sexes that at first biologists mistook the male for a growth on the female's skin. Much smaller than his mate, he attaches himself to her body and atrophies into little more than a tiny lump of sperm-producing flesh. It would be quite a stretch to make a romantic character out of an alien with a reproductive pattern like that.
How about insectoids? Octavia Butler's classic story "Bloodchild" focuses on intelligent, centipede-like females who lay their eggs in the bodies of young human men. The females show genuine affection for the human families they take under their protection; they try to remove the hatching grubs before the infants can harm their host. I once read a story about a humanoid, butterfly-like species whose females emit sexual pheromones, as many insects do. Females on this planet frequently have sexual encounters with visiting Terran men. The stimulation causes the females to lay eggs, but, sadly, the eggs are sterile because of the two species' biological incompatibility. If one of these creatures fell in love with a human male, she would have to give up her chance at natural offspring to be with him.
Come to think of it, a human character might face a major challenge in getting an alien partner to understand the concept of monogamous love. Or might the human lover choose to adjust to the alien's culture (whether it involved group marriage or something far more bizarre from our viewpoint)?