Hoshizora no Babylon -Semarikuru Cosmic Sukebe Onee-sans-

In the Universal Century of 2068, men are incredibly rare, and the protagonist, Galaxy Amanogawa, finds himself targeted by the Cosmic Race, a group of humanoid space monster girls appearing from a wormhole. These beings aim to mate with human males for reproduction. Despite Galaxy's lack of experience, with the help of the ship's captain and other heroines, they fend off the attacks. However, as a valuable male target, Galaxy remains a constant focus for the Cosmic Race, leading to continued attacks and challenges.

[Condensed from DLsite]