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Is it sexual activity
Depends on your view of what sexual activity is. Like when a doctor asks if you're "sexually active" they mean are you physically engaging in a sex related act, in which case no by that definition it's not. However it is an activity that is of a sexual nature to its core, so would say it's a nonphysical sexual activity
While it's not in-person sex, I would still call it sexual activity. If she's watching his dick live while he jerks off and cums, I would say that is sexual activity. Even if you call it something else, it's inappropriate for someone to do when they are in a serious relationship.