The Pleasure Shoppe(1976)

Original title: Hoffmann & Söhne

Mr. Hoffman sells books and provides his female customers with nice moments in a room at the back of the shop. The number of female customers grows very rapidly and when Mr. Hoffman isn't up to the demand, he calls in his two oldest sons who keep the shop going. Unfortunately, his youngest son is only interested in reading. One day, as he delivers books to one of the customers, the lady teaches him about the pleasures of sex. In order to celebrate this, the father organizes a party and all is well that ends well!