Monmusu House da! ~Bakunyuu Mamono Musume no Oppai Gyaku Rape~

The protagonist was a peaceful ordinary guy.
He was walking home peacefully and ordinarily like he always did, when suddenly he blacked out.
When he woke up again it was late at night. He was standing in the middle of the forest.
He had no idea how to get home, but there was a huge mansion.
It was a sanctuary for big breasted she-monsters...

[From DLsite English]