Rape Climax: Skinning!(1979)

Factory worker Osamu (Akira Sakai) has been despondent ever since he saw his girlfriend getting gang-raped. He has a difficult time thinking of anything else. Meanwhile, his boss asks him for a favor. Apparently, the bossman’s son is too-o-o serious about a girlfriend, Yuki (Minako Mizushima). The boss wants the relationship destroyed and asks Osamu to arrange an “unforgettable situation.” Osamu and his friends stalk the lovers to a secluded vacation house where rape delirium erupts (although Osamu isn’t capable of participating). A bit later the boss and two women show up, presumably to check on the results. But Osamu’s gang is completely unhinged from the assault and they attack the newcomers, turning the place into a sexual hell.

Official site