Doutei Yuusha wa Nakayoku Shitai

In order to defeat the Demon King... lose your virginity to your allies! The hero and his party finally reach the Demon King's castle. However, the Demon King's room is cursed with a curse that any virgin who enters will die... In order to defeat the Demon King, the virgin hero and his fellow heroines try various schemes to lose their virginity... The heroines are shocked by the hero's terrible behavior... or so you might think, but they are all in love with the hero...!? A high-tension love and brain battle romantic comedy woven by the worst hero and the cute and easy heroines!

(Source: Manga-Up; translated)