The unnamed duo, who were having intercourse for the first time, were treated in hospital for genital lesions, abrasions and bleeding.
Doctor Nguyen The Luong, deputy director of Hanoi Kidney Hospital, said that the couple were both college students. He said the male was too shy to go out and buy condoms.
Dr Luong added that the pair, who live in Vietnam, have been examined and are on antibiotics to help them recover.
According to the doctor, plastic bags, having no elasticity and lubrication, are entirely unsuitable for this purpose.
They can cause scratches and vaginal tearing and are not suitable for the protection offered by condoms.