Is it true that doctors can't tell parents their child is having sex ?

Sort of. It depends.: If the child is an older teen, and the relationship is healthy and mature (in that the couple is safe from infections and unwanted pregnancies), then the doctor will very likely keep things confidential so that the child can come to that doctor with sexually-related medical questions or issues, without worrying about her parents getting involved. The situation is different, with younger teens.

Depends: Any 18 yo in this state becomes the custodian of their medical information and i must have written permission to share any of his/her information with their parent. This includes sexual issues but is not limited to them. I cannot actually acknowledge that you have an appointment at the office. They may find out in other ways, like reading an insurance eob statement later (doesn't include dx).