Onna Darake no Isekai ni Shoukan Sareta Hi

The main character Shizuki Nakata is suddenly transported into an alternate world.
The world is called Eurupe, and only women exist in it.
With all the males gone, there is a "sacred fountain" that is used, where women bathe and gradually become pregnant.
Having never experienced intercourse, the women of Eurupe are startled at seeing a male genital.
As if possessed by a higher power, the women begin to move their hips with tears in their eyes.

[From DLsite English]