Tobira no Mukou wa

Main character is an ordinary boy who attends a vocational school. He is in relations with Saito Ume for a month already. Once she picks up some shiny seven colored crystal in the shape of a crescent, and more of such objects show up around the village. His classmates start to turn to strange nuns and mysterious gentlemen. With the help of the object protagonists successfully conquers several more girls, but situation develops in an unexpected direction.

[From vndbreview]