Otoko ga Horonda Sekai: Eiyuu Senshi no Harem World

At the age of 8, Kuuya Aragaki was caught up in a shipwrecking and transmigrated to another world. The other world seemed peaceful at first, but the countdown to its destruction had started... It was a world where men were suddenly transformed into monsters called "renma"! Kuuya was at a loss for what to do when his master, who had raised him and guided him since childhood, also turned into a renma. That's when a ship carrying a girl from the neighboring continent washed ashore in front of him...!? The only man left in the world can turn it back from the brink of destruction!

(Source: Takeshobo, translated)