In the near future, a mysterious grey mist rolled in from coastal areas, slowly covering the land. This was not ordinary mist. The mist came from a scientific experiment gone wrong, which opened doorways to another realm...

The atmosphere of this realm had altered humans in many ways. The mist morphed humans to be youthful, beautiful and elegant; and it kept them that way. Humans were often aged backwards, to biologically be 22 or younger. Our bodies could not age, or suffer permanent harm while in the mist. And our holes could not be stretched permanently; they would simply return to virgin tightness again. It made us perfect, and nearly immortal.
However, it also made human muscles became weak. The skin became more sensitive, softer, and had flawless elasticity. Human bones became impossible to break; because they could bend now. Our hip bones became able to stretch extremely wide without any damage; which also made it difficult to run or move quickly. The human sense of touch, taste, hearing, and sight were 2x more sensitive; which ultimately became a curse. This is only a small part of the many alterations made to human bodies.

The Mist also hid countless monsters, who could only survive inside the mist. But these creatures were missing a vital enzyme that they'd die without. They absorb this enzyme by repeatedly rubbing their skin in it... and they could sense the presence of this enzyme in humans.
However, females had a concentration of oestrogen in their vaginas too high for the monsters, making vaginal penetration was impossible, and males had testosterone levels far too high for them. To solve the latter, any monster that found a human male, locked them in a mysterious chrysalis which turned them into females with 100% female DNA.

There were millions of monster species, all looking for human girls to sodomize. Sometimes for days, sometimes for months; and the next one was never more than a few minutes away, waiting to thrust, brutally, into a girl's eternally-virgin-tight anus. Some lay eggs inside and some could even impregnate a girl's stomach, combining its seed directly with her DNA. Their seed was also extremely healthy for humans...

The mist continued to spread, deeper inland. Animals trapped inside, became like the monsters from the other realm, only humans became weaker and softer. It seemed inevitable that the destiny of the entire human race was to become near-immortal slaves to these monsters... As billions of naked squirming human bodies filled every street, field, parking lot, shopping mall, garden and alley; writhing from the feeling of endless sodomy at the hands of hundreds of billions of roaming monsters hunting feminine anuses to rape. The glory of mankind, reduced to nothing more than naked squirming bodies writhing in shame and humiliation at what we were forced to do... forever.

~~Idea based on “The Mist” by Stephen King; but more sexual ofc.