A person is "sexually active" when (s)he begins partaking in sexual contact
with other persons, provided that such contact is recurrent
. What is "sexual contact
"? That's another matter; but being "sexually active" does not depend on frequency of sexual activity, with whom or with how many partners it is occurring, nor what specific sex acts are being performed. For the purposes of a clinician or sex educator, the only relevant factor is the potential for exposure to infectious microbes and/or semen
Like other clinical terms used to describe behavior as objectively as possible, "sexually active" is a phrase which clinicians and educators use in evaluating whether a person is at risk for sexually transmitted diseases, HIV/AIDS, and (when fertile females are involved), pregnancy. The term applies to persons of any sexual orientation, engaged in sex with any other person of any gender or sex. However, masturbation and mutual masturbation, whether with a person of the same sex or not, are excluded.
Sexual inactivity (or "abstinence") is not the same thing as "virginity," for several reasons, among them that one can become sexually inactive after having had sexual experiences. Furthermore, and particularly for American females, guarding one's "virginity" is a project with obscure rules. Some definitions of "virginity" allow women to participate in oral (and any non-vaginal) sex, while maintaining their status as "virgins". The phrase, "sexually active" cuts through these cultural constructions, and takes square aim at the facts which matter to clinicians, disease prevention experts, and sex educators.