One of the great joys of science fiction is encountering alien races, with their weird customs and vastly different biologies. A really great alien race can make you question everything you know about humanity — but sometimes, alien races don't entirely make sense.
In particular, a lot of alien mating rituals seem guaranteed to drive their races into extinction in no time flat. Here are some alien races with highly illogical reproductive strategies, that probably wouldn't make it more than a few generations.
Kif is an Amphibiosan, a member of a species that's related to a sea cucumber — and his species has a somewhat... involved method of reproducing. In the episode "Kif Gets Knocked Up a Notch," Kif wants to settle down with Amy, and becomes pregnant — but it's not clear who the real mother is. Because, as we discover, when members of Kif's species falls in love, their skin becomes semi-permeable, and can take on DNA from anyone who touches them. Anyone. In the end — spoiler alert — we discover that Leela is the genetic parent, but Amy is the "smizmar," or person whose love made the conception possible. Kif goes back to his homeworld to spawn, and his tadpole-like offspring are clearly in danger of being eaten by random predators before they even reach the pond where they will live — for twenty whole years.
Attack the Block
We loved this movie about aliens attacking a British council estate... but the aliens' method of reproducing does seem a bit impractical. This is a species in which the males seem to greatly outweigh the number of females, and they reproduce through galaxy-spanning biologic dispersal (remember, space is very empty). They arrive on Earth as flaming meteors, but they're able to be killed by fire, impalement and blunt force trauma (their completely aphotic hides, covered in a furry cilia, are black as space, without any other protective egg or casing). The species is also seemingly blind, yet they have bioluminescent teeth, which, as seen in the film, serves no purpose but to alert prey. All of the males have to engage in a complicated hunt for the lone female, via scent alone, and it's not clear whether more than one male can impregnate her. (She would have to have a lot of babies, to make this all worthwhile.)
The sarlacc not only swallowed up Boba Fett, it's also created tons of speculation about the big sand-mouth's mating practices. And it turns out that sarlacc sex requires an intense number of coincidences. Somehow, an adult male sarlacc comes in contact with a female, where it injects sperm into
the female egg sac. The female then ejects spores into outer space where, with any luck, they will reach an inhabited planet and attach itself to a creature of a certain size as a parasite. Eventually, the sarlacc detaches, and fends for itself as a predatory worm, challenging larger and larger creatures as it grows. If it is not killed and eaten, the sarlacc will eventually encounter a creature large enough to eat it, where it again lives as a parasite (varying accounts say it will kill its host outright, so the creature must be very large to avoid decaying immediately) until it grows large enough to eat its host from within — at this stage, it will develop its beak-like"tongue" and tentacles. When complete, the sarlacc is ready to dig its burrow and grow roots, where it develops a secondary, encircling mouth, and becomes a pit.
Similarly, in "The Twin Dilemma," the alien caterpillar Mestor planned to detonate a planet, in hopes the shock wave would carry its eggs across the galaxy. What is it with aliens sending their eggs into space? Remember: Space is very, very empty.
In this book series, it appears that Yeerk reproduction involves the fusion of three individual Yeerks into a single organism -– a process none of the "parents" survive. The adult Yeerk eventually breaks apart into hundreds of offspring "grubs" –- Yeerks have no living parents, so the grubs, who who are parasites but also need "Kandrona Rays" to survive, are left to fend for themselves.
Apparently there's more to this species than just Kevin Spacey staring into space in weird sunglasses. The K-Paxians, or "dremers," as the book version calls them, have a method of reproducing that the book describes as "extremely unpleasant", involving intense pain and horrible odors. As K-Pax explains, "It's more like having your gonads caught in a vise, except that we feel it all over. You see, on K-PAX pain is more general, and to make matters worse it is associated with something like your nausea, accompanied by a bad smell. The moment of climax is like being kicked in the stomach and falling into a pool of mot shit."
According to some of the books, the Andorians have four genders: Zhen, Shen, Chan and Thaan -– and each is needed to form a telepathic bond in order to reproduce. The complications of this quadrigender paradigm has actually caused the species to be driven to near extinction.
The Xenomorphs in the Alien movies have a weird method of reproduction, if you think about it — first of all, they need to plant eggs inside of a host, like some wasp species, but compatible hosts are probably a lot harder to find in space than in a terrestrial environment. And then there's the fact that they apparently take some DNA from the creatures they plant eggs in, hence the Xenomorph dog. How different can your DNA be, before the Xenomorphs aren't able to make use of it?
Killer Klowns from Outer Space
The aliens seem to reproduce through spores resembling popcorn – which they also use as rifle ammo and engine fuel.
Invasion of the Body Snatchers
The "pods" travel to a planet, depopulate it, and then move on to the next world — which seems like a lot of work, with probably lengthy periods between planets. In the original novel that all the movies are based on, it's spelled out that the "pod people" only live for five years, after which the world is left depopulated and they have to find a new planet to take over. (Also, in the original book, the aliens are pretty easy to scare off.) As one of the pod people explains, "The duplication isn't perfect. And can't be. It's like the artificial compounds nuclear physicists are fooling with: unstable, unable to hold their form. We can't live, Miles. The last of us will be dead... in five years at the most."
Star Trek again
And then... there's Pon Farr, the Vulcan mating ritual. It's true that the Vulcans live much, much longer than humans, so a once-every-seven-years mating process could work. Although the need to have mortal combat with any romantic rivals as part of the foreplay could reduce the fertility rate quite a bit.
Update: People have mentioned in comments the possibility that Vulcans can reproduce outside of Pon Farr, something that's never been mentioned on screen. Here's how Spock describes Pon Farr in "Amok Time":
It has to do with biology... Vulcan biology... The birds and the bees are not Vulcans, captain. If they were, if any creature as proudly logical as us were to have their logic ripped from them as this time does to us... How do Vulcans choose their mates? Haven't you wondered? It is not [done logically]. We shield it with ritual and customs shrouded in antiquity. The humans have no conception. It strips our minds from us. Bring a madness which rips away our veneer of civilization. It is the Pon Farr, the time of mating. There are precedents in nature, Captain. The giant eel birds of Regulus Five: Once each eleven years they must return to the caverns in which they hatched. On your Earth, salmon... they must return to that one stream where they were born to spawn, or to die in trying... We are driven by forces we can't control, to return home and take a wife. Or die.
We debated the logistics of Pon Farr a lot in our staff meeting, and concluded that it would probably be okay, as long as you had a society where nothing bad ever happened. But it would be highly impractical for a society recovering from a cataclysm like war, pandemic... or, say, the destruction of their planet.
Also, there are the Ocampans on Star Trek: Voyager, who are only fertile once in their short lifespan.
Thanks to Annalee, Alasdair and Lauren for suggestions!