Gakuen Monogatari '93 ~The Instinct~

Millions of light years away from Earth, on a planet called Cicero, there existed a highly advanced civilization. However, it was all but destroyed by a life form known as "Barius", a parasitic creature that caused living beings to completely give up to their sexual instinct and innate aggressive tendencies, effectively turning them into monsters. To avoid global contamination, the inhabitants of Cicero left their planet. The remaining creatures, infected with Barius, used a capsule capable of space travel, and eventually showed up in our galaxy. The capsule containing ferocious monsters is nearing the Earth. Unaware of this unimaginable danger, the protagonist of the game, a young teacher in a girls high school, thinks of nothing but sexual pleasures...

[From Moby Games]