Hyakumannengo no Sekai ~Ashutachi no Owaranu Seiyoku no Giseisha~

Man has evolved into tentacle monsters who can get a woman pregnant with 100% certainty. Is there any future for mankind?

Technological progress has halted, and civilization has regressed into a more primitive state.
Earth is covered mostly by wastelands and wilderness. Humans are struggling to survive.
In this environment, some people have evolved into a more monstrous form. They're called sub humans.
Those who retain their human qualities are called the true breed.
The sub humans rise up against the pure breed, and impregnate their women with their offspring.
The true breed's numbers dwindle until their very existence is in danger.

[From DLsite English]