トワイライトゾーン VOL. 4 特別編

Aliens have invaded Earth. Seemingly impervious to damage, they took control of the planet. Only a few partisan organizations dared to oppose their rule. All the aliens look like young and attractive human women, and soon it was found out that they have a weakness to sperm. Thus, a young Japanese man and his few male allies embark on a quest to defeat the aliens by having sex with them.

[From mobygames]