(N-POV)
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Before you make assumptions when reading the exposition, this isn't about alien rape.
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*Swooshing*
In the kind of distant future, the earth is under threat from an intergalactic species of xenomorph. This species cannot reproduce on its own and relies on others to host its offspring. This very species is approaching earth with an invading army.
These aliens are infamous for making species extinct across the galaxy. It brings them great sexual satisfaction to dominate and obliterate other inferior species. They turn all of the females into breeding slaves and kill off all the males. They then reproduce with the females until they have made a new generation and a bit extra. Then kill off the females as well. Since the aggressive nature of the aliens rapidly ruptures the insides of their victims, they become broken and useless after being used several times.
...Holy fuck.
They move on going from planet to planet every generation. There was hard evidence for this on earth. This is obviously not a good thing that people were ok with. So world governments scrambled to defend the planet. Even though it was practically assured that humanity would lose with its inferior weaponry. A species who relies on conquering surely has very advanced weapons.
However, there was a saving grace. The aliens would only bother breeding with viable subjects. This meant they needed a substantial amount of females with unfertilized eggs. I think you can see where this is going...
The solution was simple, although hella lewd and a hentai plot if I've ever written one. Which I have. The solution was to get every female of certain age on the planet pregnant. If they did this then the reason for invasion would be negated and the invaders would leave to the next planet. A lot like herd immunity.
The aliens had somewhat of a time limit too to find a viable host species for their next generation. Since they were getting old. Simply waiting nine months was far too long.
Basically, on Earth, women were given a choice to either pick someone to copulate with or be paired up. If it really went that far then artificial insemination was used, but could not possibly be done fast enough to the whole planet.
Right now, a little over a week is what humanity has. And organizing an alternative plan took time. There was no other apparent solution. The invasion force was inbound and sexually frustrated. A recipe for humanity's destruction. There wasn't enough time to fuck around. Only time to fuck. (Lol I actually typed that XD.)
Many women, although understandably reluctant, went along with it. Since the alternative was to be raped and forcefully impregnated by aliens. Then birth their horrid offspring. And nobody likes that. Not even me, and I'm the author! Notice how that isn't what this scenario is about. Anyway, let's get to the 'anything but that' bit.