Can a doctor tell your parents about your sexual activity if you are under 18?
Yes. Its that simple. * No, not in the general context. Federal
regulation Title X supports a minors rights to privacy when it
pertains to issues such as contraception, treatment of STD, etc.
(the exception in some states is abortion). 30 US states by
statutory law protect the minors rights to privacy when it relates
to medical care such as cited in the question. The US Supreme Court
has continued to rule that the US Constitution affords a minor the
same privacy rights in such issues as it does an adult regardless
of the laws of the state in which the minor resides. The best
choice would be for the minor to discuss such matters with their
parents. In lieu of such, the legal community will uphold said
minor's legal rights to privacy in such issues. The medical
community tends to have the same view with the exception of life
threatening situations where it is believed that the minor is not
able to make an informed decision.