Lune - Youkoso! Sukebe Elf no Mori e

Alfheim, a world of elves where women only exist.
While a war is ongoing between the two factions, the factions of the cast out dark elves and the faction consisting of elves and high elves, the holy tree, the source and mother to all elves, has started to lose its power and it stopped giving birth to new elves. However, it is found that a man from another world can save their tribes.

And it was not too long the protagonist is now brought to the world of Elves... However, since both elven factions have lost many members of their tribes the only way for them to multiply and win the war, was to bear the protagonist's children.

And so both factions approach him to make babies for their own sakes, the protagonist falters on what to do when in between warring elven factions