Shojokan Hatsutaiken o Ryoujoku Sareta Otome-tachi

In Tokyo disease "XUM" with no cure starts to spread rapidly. Main character decides that he is infected. Internet rumors say that "XUM" is actually a curse that is lifted upon violating three virgin girls. Protagonist finds three virgin girls, but continues to violate them even after their virginity loss, just in case.

[From vndbreview]