I've just finished rereading DR. TATIANA'S SEX ADVICE TO ALL CREATION, by evolutionary biologist Olivia Judson. You should all get this book! In the format of an advice column for animals, plants, and microbes with romantic and sexual quandaries, the author imparts densely packed information on reproduction throughout the natural world in a most entertaining style. She explores the evolutionary advantages of patterns such as promiscuity, monogamy, infidelity, homosexuality, hermaphroditism, parthenogenesis, and the most aberrant behavior of all, complete asexuality. Did you know that many animals change sex over their lifetimes, depending on environmental or social conditions, or that in some species the sex of the young is determined by factors such as water temperature rather than genetics? Have you heard about the hermaphroditic snails who have penises on their heads? Lionesses in heat copulate at least once every half-hour over four or five days. In many species of birds thought to mate monogamously, DNA tests of chicks have shown that infidelity runs rampant. Despite the popular perception of males as promiscuous and females as monogamous, polyandry is far from unknown in the animal kingdom. Some bird females will have several mates, each on a separate nest incubating his own clutch of eggs. And we've all heard of the male spiders and preying mantises who present the ultimate gift to their mates to ensure the robustness of their children: She devours him (literally) after fertilization. Some male insects even copulate more vigorously with their heads bitten off. (Passing up the easy punch line. . . .) Strangest of all, some slime molds have over 500 sexes. (No, that doesn't mean they have to get 500 individuals together to mate. It simply means they have that many distinct kinds of reproductive cells. It still requires only two to produce a new organism.) I was surprised to learn that sex and reproduction don't necessarily go together. “Sex” basically means a method of exchanging genetic material between individuals. That exchange may be separated in time from the creation of new individuals (e.g., an organism might trade DNA with another one now and generate offspring by budding later).
Suppose your human hero or heroine forms an emotional attachment to an alien who isn't humanoid or even mammalian in reproductive biology but something much wilder, such as one of those species. If the couple doesn't mind being unable to have children together, might they share sexual gratification anyway? Or would it feel too much like bestiality? How dissimilar would they have to be, physically, to trigger that perception? At some point of difference they might have to confine themselves to emotional bonding, as "soul mates." Or they might just be intimate on the level of friendship. I'm reminded of the STAR TREK episode in which the blind telepath bonds with the medusoid whose physical form is so alien (or so dazzling) that no human being can look at him/it without being struck mad. That bond must be (in the words of the introduction to the TV show BEAUTY AND THE BEAST) "stronger than friendship or love," yet it's completely nonphysical.