A good bit of imagination has gone into attempts to represent close encounters of a sexual kind. Major authors, such as Ursula K. Le Guin, Philip Jose Farmer, and Octavia Butler, have written critically-acclaimed novels in which deep emotional, intimate, and sexual relations between humans and aliens form the central narrative. Significant episodes of the various Star Trek series have also shown intimate relations among aliens — sometimes between humans and aliens, sometimes among different alien species (such as the relationship between the Klingon Whorf and the Trill Jadzia Dax in Deep Space Nine). Star Trek’s own Mr. Spock was the product of a marriage between a human female and a male alien from the fictional planet Vulcan.
The human/alien sex trope typically assumes that an alien species would reproduce in a manner that’s compatible with how humans reproduce, but that can be a dangerous assumption, judging by the way many species on earth reproduce. The male of the deep sea anglerfish, for example, attaches to the female permanently, fusing with her blood stream and gradually atrophying until they are just a pair of gonads that release sperm into the female in response to hormonal cues in her blood, a process known as sexual parasitism. Other species, such as the New Mexico whiptail, are an all-female species. Adult female New Mexico whiptails reproduce solely through parthenogenesis (a type of asexual reproduction in which a female gamete or egg cell develops into an individual without fertilization), laying unfertilized eggs that develop into other female whiptails.
There are organisms without distinctive male and female forms. The black mold Rhizopus nigricans, displays an unusual form of reproduction known as “heterothallism.” This species of fungus requires two organisms are for fertilization and replication to take place. However, the two sexes are physically indistinguishable. There are no constant differences between members of opposite mating groups other than their reciprocal behavior when crossed. Thus, it is impossible to designate one form of the black mold as male and the other as female. Customarily the complementary groups are labeled merely “+” and “-” for convenience during experiments.
If some species have no distinctive examples of male and female forms, others have more than two sexes. Some species have one female and two male genders include red deer who have two male morphs (distinct forms of an organism or species), one with antlers and one without, known as hummels or notts, as well as several species of fish such as plainfin midshipman fish and coho salmon. Others have one female and three male genders, including bluegills, where four distinct size and color classes exhibit different social and reproductive behaviors, the spotted European wrasse (Symphodus ocellatus), the Oreochromis mossambicus cichlid, and the ornate tree lizard (Urosaurus ornatus).1 Slime mold has thirteen sexes, while the fungus Schizophyllum commune has over 28,000 sexes.
Other organisms lack specific maleness and femaleness, but exhibit an alternating or intermediate condition. For example, simultaneous hermaphrodites possess at once both female and male sex organs. Ovaries and testes are present together in the same individual. Matings occur in pairs, with each partner serving both sexual roles at the same time. Planarians, earthworms, sponges and snails fall into this category, as well as a few among more highly evolved vertebrates are known, such as the belted flamefish (Serranus subltgarius). Some simultaneous hermaphrodites form harems, in which a single male supervises a school of females. If the male is killed, the dominant female in the harem transitions and takes his place. In such harems, the male is usually responsible for defending the school from invaders and protecting his territory to ensure that his flock does not wander astray or encounter potential dangers.
There are also species where individuals start life as one sex and finish it as another, called sequential hermaphrodites. There are two types of sequential hermaphrodites. Animals which are born male with the ability to become female exhibit a trait of protandry. Protogyny, on the other hand, is a trait in which animals are born as females, with the ability to become males later in life. Depending on the species, simultaneous hermaphrodites may change sex only once, or they may be able to flip back and forth between genders several times. Oysters, for example, are born as males, then spend the rest of their lives switching back and forth between male and female in irregular cycles a few months long.
Given these tremendous potential biological differences, would or could copulation be possible at all between humans and extraterrestrials?
There are well-documented cases of attempted and successful sex between different species. These include dolphins and humans. seals and penguins, otters and seals, sheep and deer, orangutans and humans, and humans with various farm animals. This suggests that, if humans were to mingle socially with alien races, interspecies sexual contacts are at least possible.
Even if such activity were possible, how likely would it be? Could humankind and an alien race derive sexual pleasure from mutual physical encounters?
Suppose, for example, we discovered an alien race that is sequential hermaphroditic. In this society, individuals spend their early life neither male or female, then attain puberty and enter their first sexually active phase as functioning males. After a certain amount of time, latent ovaries within them ripen into maturity, and the individual, now considered an adult, spends the remainder of its life as a female.
In such a society, monogamous marriage as we know it would be impossible. Husbands would change into wives, and males would be too immature psychologically to be treated as anything other than young lovers. Since all fertile middle-aged females in a family in theory could mate with any or all male children, it’s likely there would be complex incest prohibitions. To offset the negative effects of inbreeding, exchanges of matriarchs would occur between families. Love as humans understand it probably would not exist. In such a society, females would likely have strong affective and familial non-sexual ties with other females. Human concepts of male/female romantic relationships would be quite incomprehensible to them. Even if their sexual organs were compatible with those of a human, and neither species found the physical appearance of the other repellant, the sensitivities of the alien race would likely be such that there wouldn’t be any desire on their part to have any type of sexual contact with humans.
1. See Roughgarden, Joan (2004). Evolution’s Rainbow: Diversity, Gender, and Sexuality in Nature and People. University of California Press. ISBN 0-520-24073-1 Especially chapter 6, Multiple Gender Families, pp. 75–105.
The Sex Is Out of This World: Essays on the Carnal Side of Science Fiction, edited by Sherry Ginn, Michael G. Cornelius, Donald E. Palumbo, and C.W. Sullivan III
Alien Sex: From Ming the Merciless to “The Lovers“, David Lumb and Jonathan Alexander, Los Angeles Review of Books
“Alien Sex Acts in Feminist Science Fiction: Heuristic Models for Thinking a Feminist Future of Desire“. Alcena Madeline Davis Rogan, PMLA, Vol. 119, No. 3, Special Topic: Science Fiction and Literary Studies: The Next Millennium (May, 2004), pp. 442-456