With every male they'd ever known dead, every friend, brother and son, most of the planet spiraled into complete depression. The world also lost most of the skilled labor and half of the work force. All governments underwent radical restructuring. In many places they fell into total anarchy. the greatest depression of all time followed, and humanity fell into a new dark age. In this environment most women committed suicide. Most of the remnant became consumed with revenge. They devoted themselves to rebuilding a small society and acquiring the science to strike back at the aliens. A small branch of them, half mad by now, used the technology to wipe out most of the other cultures because of their ridiculous new forms of discrimination. They never did develop the technology to reach the aliens. Just barely enough of the population managed to reproduce. Humanity limped on, but scattered and lost as they were, civilization and education were not sufficiently passed on. Humanity lost all technology and reverted to primitive nature based religions. Four thousand years later a new society had evolved to the point where they could travel across the continents and wonder about history. By some coincidence Americans had died out completely, so when America was once again discovered the explorers were perplexed to find an enormous carving in the side of a mountain which read(in the dead language of English) "Fuck those alien assholes. If some half decent species should discover our planet, we beg you to have the decency to hunt down the bastards that did this and avenge us".
Turns out that message worked pretty well. There was an intergalactic vigilante, sort of a Rorschach kinda guy, who found it and managed to kill the asshole species with a virus. The rest of the galaxy, used to this kind of stupid crap by now, pretty must just accepted it as natural evolution. Anyone dumb enough to wipe out half of a species like that was just asking for it.
The End
I guess it's a good thing CYOC usually isn't realistic.