Itou Yuuichi

A regular Japanese young man, Yuuichi was aimlessly carousing around the streets wondering when his luck would change, hoping to a score girlfriend soon; when suddenly a strange-looking young girl appeared in front of him, Yuuchi thought the girl was a cosplayer or something, the way she was dressed, but Yuuichi has no time to think any further as he spirited away to a completely different world.

After a few mishaps Yuuichi is captured by what appears to be elves, the strange girl that spirited him away introduces herself as the elder Nol, who explains to Yuuichi their female-only populace is diminishing, their Holy Tree which once gave them elven offspring is weakening and can no longer produce new children; so Nol proposes Yuuichi conceive more elves with the women of their village, Yuuichi accepts if Nol takes him back to his world.

Spoiler

Yuuichi soon becomes another point of contest in the war between Elfs and Dark Elfs, as the first sexual contact with a male awakens the libido those elves never had prior in their very long lives, and it is proven his semen can also fill up their magical reserves, so each side wants to keep Yuuichi only for themselves; however, Yuuichi's surprisingly kind and selfless nature is what wins the elves over, often disarming their willingness to fight each other, thus they eventually procreate out of sincere love.