Ren'ai Byou

Population starts to decline rapidly due to unknown reasons, and many people lose hope for tomorrow. With time a rumor starts to circulate that people in love have much bigger mortality rate than those who aren't.
Main character loses a bet with a bad friend and confesses to his classmate, and their shy relations start to develop. But with rumors circulating a shadow of death hangs in the air.

[From vndbreview]