Sexual activity is a phrase which has a (now) very vague and indeterminate meaning. In the past, "sexual activity" was a euphemism for coitus (the insertion of the penis into the vagina). Other forms of physical touch (kissing, touching or massaging of various parts of the body, etc.) were not considered to be "sexual", rather they were either considered to be sensual or merely to be exhibitions of social interactions.
Currently, the right-wing antisexuals and sexophobes (and the third-wave feminists) have redefined "sexual activity" in such a way that the term may be applied to almost any activity, whether involving direct physical contact or not, and involving any part of the body.
This has been a boon to lawmakers (and the prison-industrial complex) as it allows more people to be convicted of "sexual crimes" -- almost at the whim of the prosecuting attorneys - if the "state of mind" of the person accused involved any kind of sexual imaginings.