Mu teenage son came home one night with a girl he obviously intended to sleep with. I really didn' know what to do. In the end I rang up the girl's mother, and then felt guilty. How should I have coped with the situation? Sex and drugs are two common areas of conflict between parents and teenage children. DR WENDY GREENGROSS , marriage guidance consultant. DR JAMES HEMMING , educational psychologist, and DR JOHN PAYNE , director of a student health service. discuss three particular problems with the parents involved. In the chair JEAN METCALFE Producer HUGH PURCELL