At the end of the Earth's life, a humanity seeks out a new world to settle upon, colonists "sleep" in suspended animation as their ship automatically navigates them toward what would be their new home.
Mankind has long since perfected themselves with the advances made in biotechnology, colonists were modified even further in order to optimize their chances of successfully settling their species on the strange new world they were being sent to.
Their designer genes equip them for using taboo sex acts like incest and pedophilia to populate the planet, and to sustain themselves with vampirism and cannibalism should they have to, they had hoped that such modifications would not be necessary, but it was fortunate that they had made preparations for the worst.
The female colonists were all artificially impregnated in sleep so that after they arrived they would give birth to boys who could then help to populate the species by having sex with their own mothers should there be a lack of adult males for them to partner up with.
The necessity of these sorts of modifications turned out to be much greater than first thought as the colony ship crashed upon it's destination.
All but one of the crew perished before waking up and the only survivor was a female who would need to use her son to build the human presence upon the planet.
Food is almost non-existent in the particular region of the planet surrounding the crash site, water is almost as scarce, much of whatever supplies were brought to the planet was lost along with most of the ship itself, being scattered across the planet's surface in various wreck sites.
She had to resort to surviving off of the bodies left by her fallen crewmates. There was more than enough human flesh to feed her hunger, and blood to quench her thirst while she carried her fetus to term, eventually giving birth to her son.
Almost immediately she began sexually abusing her newborn child, as this was a way to normalize them to having relations with their close family members and overcome any psychological barriers to doing so, it may have been unpleasant (initially), but she knew it was something that needed to be done for the sake of the survival of the human species upon this world.
The fact that she grew to greatly enjoy stroking and sucking her newborn's tiny peen as her psychological barriers of innate revulsion gave way to a new paraphilic perversity.
Now human repopulation was a secondary concern and she would engage in all manner of sex acts together with her son every night, it was a nightly ritual that she had become obsessed with, almost addicted to.
The minds of the colonists were also adapted to regularly entering into such relationships should they become a necessity, but as a result once such taboos are broken, these kinds of sexual activities become very pleasurable and could even become a normal part of life for them.
The genetic blueprint of colonists' bodies were adapted for this as well; Children were made to be as capable of having sex with adults as they could be at any age, Colonist DNA was carefully purged of dormant defective traits that could be brought to the surface through incest, all this and more was thoughtfully planned out in advance in order to ensure the greatest chance of humanity flourishing in their new home.
So the boy grew up making love to his mother every night, doing more with each other each time as he became a more capable lover, and eventually he would get her pregnant with the first of the many children they would bear together.
The mother would instruct her firstborn son to carry on with his daughters in the way she had with him, and from the day they were born they were known carnally by their father.
The matriarch would set herself upon her sons in the same way, from their first days of life they were known by their mother. Of course, she continued the relations with the familial patriarch.
Of course, they were also encouraged to lay with their siblings in many ways, just as they had with their parents, though it turned out that the encouragement wasn't required, the siblings were very eager to have sex with each other all of their own volition, and as a result incest became an ordinary part of their family life.
Many offspring were produced from these incestuous and pedophilic unions, the family continued these traditions as they expanded the clan even further, until they became the beginnings of a civilization where pedophilia and incest were the norm.
Sex itself was very unproblematic for the colonists as they were made with an immune system that safeguarded against all pathogens, and sexual activity was soon to have no taboos associated with it at all.
People would fuck their baby daughters in public, and they might even offer up their little girl's ass to strangers if they indicated that they were interested.
Despite having been regressed back to the stone age by the loss of all data and technology in the initial crash, the descendants of the first colonists quickly rediscovered lost technology and expanded their territory far and wide, eventually splitting off into tribes, which become countries, which become empires.
All this was helped along by the parts of their colony ship's wreck that they discovered along the way, they even found the descendants of some of the other "sleepers" who had survived the crash, but who landed in other distant areas, they had an easier time of it all considering their crash site had more than one initial survivor and far more access to food and water, but somehow they all fell short despite having had such a better start these many tribes were integrated into one with our protagonists over time.
Whether their traditional culture of perversions would survive or die out in time is another story.
This concept sets up all kinds of sexual scenarios, and it could be modified from this form if the whole "After Earth" idea isn't satisfying enough, you could go from something futuristic to something historical by making it about a pregnant woman being the catalyst of civilization emerging from largely uninhabited regions of Earth sometime in the past, perhaps during ancient or prehistorical times (you could introduce zoophilia into the story by having some of the first domesticated animals become tamed through bestiality).
It's a concept that has been bounced around here for awhile and I'd like to see someone do something with it, perhaps some artfag could make a comic out of the idea?