Youkoso! Sukebe Elf no Mori e (Series)

Alfheim, a world of elves where women only exist. The world Shrine, the source of all the elves' magical power, has started to fade and the race was in danger of extinction. Then there is a prophecy that a man from another world is destined to be the savior of the elven race. And it was not too long the hero is now brought to the world of Elves... The elves ruled they must survive by child-making with the hero. But many of the other elves refused this vulgar decision and so the elves divided into groups: Those who disdain child-making with a man and those who choose to accept the fact. But those who accepted have discovered something: It turns out if they have sex with the hero and acquire semen into their bodies, their magic powers increases. So they proceed to milk him dry... (Source: DLsite)