When Women Had Tails (1970) on Netflix

A bunch of 7 orphan cavemen grow up on a little island all by themselves. After a fire burns all vegetation they set out to find a new place to live. Here, one day they trap a strange animal, looking very similar to them, only softer and with longer hair. She (Filli) feels attracted to one of the brothers (Ulli) and convinces him, that a certain kind of playing with each other is far more satisfying than just eating her. He hides her and serves the other cavemen a goat. Soon Ulli and Filli are playing the Johnson-Game quite frequently. But the others find out about this and want to see what's so great with "johnsonning". After they all force Filli to show them the game too, she manages to flee with Ulli. Being pursued by the 6 men of Ulli's clan, they meet the rest of Filli's clan: A tribe of about 150 unsatisfied women, who just love to have new men around for playing.