Science Officer Srock: “Fascinating.”
Ship’s Physician Bonehead: “You can say that again.”
Srock: “Very well, Doctor. Fasc…”
Bonehead, Captain Kirkland: “STOP!!”
Bonehead: “Must you always be so literal?”
Srock: “Surely, by now, Doctor, you understand that mine is not a particularly aquatic species.”
Bonehead: “… whut?”
Srock: “And I fail to see how the fate of this civilization hinged in any way on my proximity to a littoral zone.”
Bonehead: “O .. my ..”
Kirkland: “Enough. What did happen to this civilization, Srock? It appears to have achieved an Index 21 industrialization level, and most of the infrastructure is intact. There’s no sign of war or pestilence. It’s as if everybody just walked away”
Srock: “Indeed, Captain, the infrastructure is remarkably intact, almost as if the people who left did all they could to pack it away carefully before leaving. Or, more precisely, before dying out, which appears to be what took place. They left plentiful records, on solid, analog, and digital media, and I’ve spent much of this away mission studying what I could, and uploading to the ship what I didn’t have time for.”
Bonehead: “They died out? Of what?”
Srock: “According to the evidence, Doctor, they died out from a society-wide decision to cease reproducing.”
Bonehead: “What? That’s crazy!”
Srock: “That, Doctor, is characteristically unanalytical of you. It is also not unprecedented, the Shaker community on Earth made a similar decision, and suffered a similar fate.
“The species that created this civilization was humanoid, and, alas, evolved the same unfortunate strategy of using copulatory behavior, which is utterly selfish in motivation, to drive social bonding and other forms of community behavior. It would appear that the same civil engineer designed the bodies and minds of this species and humans.”
Kirkland: “How so? And do I want to know?”
Srock: “Surely, Captain, you’re familiar with the old Earth trope that claims that the human body could only have been designed by a civil engineer, because only a civil engineer could envisage, and justify the construction of, a sewer system that passes through a playground.
“And, on this world, the playground became all-important. And because it had become all-important, all the self-centeredness associated with it broke through. Infidelity, promiscuity, pedophily, rape, and other forms of violence induced by the pairing of unsuited, and indisciplined, individuals through the temptations of the copulatory playground, all became prominent. Medical enhancements of primary sex characteristics were all the rage, usually by one gender at the expense of the other.
“The population became so focused on the pleasures and perils of procreation as recreation that other forms of selfish asocial and antisocial behavior were largely ignored. In public and private life alike, no amount of corruption, cronyism, theft, rudeness, or incompetence mattered at all, so long as the populace accepted the copulatory behavior of the individuals involved, either for their propriety or their entertainment value, often both. And, as with all forms of mob rule, there was no consistency in the patterns of approval. Politicians in favor could rape with impunity, whereas others, not in favor, could have their careers extinguished instantaneously for once touching their spouses in the wrong place at the wrong time. Allegations of copulatory misconduct – accurate, inaccurate, or fictional – became the principal weapons for contesting status among individuals at all levels of society, and the principal source of income for those whose careers it became to disseminate the allegations, no other ‘news’ being profitable, which became the only definition of ‘worthwhile’. Meanwhile, the social cohesion necessary to sustain complex societies broke down, and with it the societies themselves, along with the critically-important infrastructure required to support them.
“The situation became so extreme that it engendered an extreme reaction. A party rose up that overthrew the status quo ante planet-wide, and replaced it with one that banned all copulatory activity, on pain of summary execution of any individual indulging in it. It focused attention instead on preventing the imminent collapse of civilization, and its 11th-hour successes at doing so kept it in power, and its strictures in place.”
Bonehead: “They saved a society so they could leave it to the frogs and toads? How stupid could they get?”
Srock: “As I said, Doctor, it was an extreme reaction. A quick perusal of human history tells me that your surprise at this is utterly inconsistent with the behavior of your own species.”
Kirkland: “But humans are still here, Mr. Srock, and that means we’ve generally been able to spot our bottlenecks in time and somehow wriggle through them. How is it that this civilization didn’t figure out that it was headed for destruction?”
Srock: “It evidently preferred destruction to calamity, Captain. The revolutionary party consisted exclusively of females, whereas the society they overthrew was very much male dominated. Once in power, these females focused on their mission, and their careers within that mission, 
and were unwilling to waste time and expense on pregnancy, child-bearing, and child rearing. ‘Motherhood’ became a synonym for ‘slave’, a status that no female would accept even it if had been possible under the copulation ban. And they were totally unwilling to delegate child-rearing tasks to males, whom they detested and did their best to exterminate.”
Bonehead: “Well, then, medical technology …”
Srock: “Do you recall, Doctor, that I reported the deterioration of societal infrastructure? This included the gutting of all forms of genetic research and technology-assisted reproduction, what you humans used to refer to, rather crudely, as ‘test tube babies’. Long before the revolution, the society had rejected all ‘genetically modified organisms’ and any research associated with such, and had likewise rejected all forms of technological intervention with the reproductive process – abortions, for example, were banned – probably because such interventions were seen as threats to ‘male prerogatives’. By the time the revolutionary society recognized its peril, if indeed it ever did, the scientific knowledge base and medical-industrial capacity needed for technological solutions was well below critical mass, and could not be resuscitated in time.”
Kirkland: “And so they died out. This is the way the world ends, not with a bang but a whimper.”
Srock: “I doubt, Captain, that these revolutionaries, even in their last hours, permitted themselves so much as that. But there is one thing that can be said in their favor.”
Kirkland: “I shudder to ask, but … what?”
Srock: “Their planet is environmentally pristine.”
Bonehead: “Gah!”
Kirkland: “Well, Mr. Srock, I’m sure that a multitude of Federation planetary systems will jump at the chance to muck up that environment in their memory. Mr. Spot? Three to beam up.”