The Seducers (1969) on Netflix

Attempting to cure her mentally handicapped son's virginity, his mother and several friends embark on a cruise which strands them on nearly deserted island where lust and murder are on the menu!

Ulla, a prostitute is hired by the despicable Mudy, mother of the shy and mentally disturbed 20-year-old Tony with a tendency toward pyromania. She is invited on a sea cruise where she is meant to "take" his virginity. Also invited on the cruise are the provocative Paula and her husband Aldo, who are constantly striving to win the favor of a wealthy woman in hope of obtaining an oil concession. Despite her efforts, Ulla has no effect on the young man until the yacht stops on a Mediterranean island inhabited only by a goat herder and his wife, Beba. Tony is attracted to her, but little by little his mental disorders arise, and the story ends in tragedy.